Your lights and or dimmer are reacting to the common neutral. LED lights are very sensitive to the dimmer. Many are electronic and common neutral can cause issues.

No pic showing so just basic overview. Can test with reg incandescent light to see if still get flicker. If no, then your fan may need its own independent power....no shared neutral.

You said you have red/black wire setup...do they have their own circuit? (breaker). Guessing no and that can be a source of issue as well.

This is a relatively common but annoying problem with cheap LED drivers. Inductive loads (such as fan motors) on the same circuit as an LED driver can cause the capacitors within the driver to discharge when the inductive load is shut off. This results in a brief flash of the light. It is not dangerous, just annoying.

As a side suggestion and question, how deep of a reno is this? Are the walls back to the studs and how hard would it be to run new wire? I highly suggest running 2 new circuits, back to the panel. First off you really should have bathroom outlets on a separate circuit from the lights. It should also be a 20 amp circuit and a GFCI outlet, this is standard NEC code. If you think the lights flickering when the fan turns off is annoying, try plugging in a hair dryer into your outlet shared by the LED lights… prepare to be extra annoyed. I would then add another 15 amp circuit for the general lights and keep the existing circuit for just the fan. I don’t believe code requires the fan and lights to be on separate circuits but it will solve the issue you are having and running 1 circuit vs 2 to the same location is fairly minimal additional work.

> I would then add another 15 amp circuit for the general lights and keep the existing circuit for just the fan.

This is a serious waste of a circuit. Fans pull little current. Get better LEDs.

Lights/fans and countertop outlets in a bathroom can be on the same GFCI protected circuit if that circuit feeds only that bathroom. If countertop outlets from multiple bathrooms are on the same circuit, then all other power in the bathrooms need to be on a different circuit(s). Of course, this is current code. Wiring that existed prior to this code adoption may certainly be different. In my 1959 house, the dishwasher was the load on the bathroom countertop GFCI. That was fun to figure out.

This is definitely false for new code, sorry. An old house is grandfathered in, new installs are not. It’s a moot point regardless even if you ignore code it’s just bad practice. If you plug in a hairdryer into a lighting circuit with the best LED lights it WILL flicker. No questions about it.

Regardless the code says, outlet is on a different circuit from lights, so 20amp for outlets and 20 for lights IF shared by the fan, but again why share? Run a 15 for lights and 15 for fan. I’m not sure what you mean by a “waste of an circuit” unless OP has limited space in the main panel the only waste is 50-100 feet of 14-2 which is honestly around the price of running a single 12-2…

edit link to relevant code, granted from spruce but i’m sure you can find the relevant NEC code https://www.thespruce.com/easily-wire-a-bathroom-1152676

>This is definitely false for new code, sorry.

I'm not reading your link at 1am. Tell me what I wrote that's wrong. I'm aware that old installs are grandfathered in, I stated that much. Updating your out of date electrical isn't required when doing fixture replacement. I won't argue that it shouldn't be brought up to code, because it should be, but nothing I stated is wrong.

I say a waste of a circuit because a bathroom fan might use 60 watts. Why on earth would you use an entire circuit for the same current draw as an incandescent light bulb? The only time you'd want to run a new circuit here is if the entire bathroom is on a single circuit so the fan and countertop outlets are sharing the same circuit. But there is ZERO reason to separate a fan from your lighting. Either update your LEDs or fan.

Additionally, lots of houses have smaller panels, especially older houses where the same circuit powers the bathroom and additional areas. Newer houses still run out of space on a 200A 30 space panel.

Edit: Figuring maybe you're talking about lighting and outlets being required to be on separate circuits in new code.

210.11(C)(3) Bathroom Branch Circuits. In addition to the number of branch circuits required by other parts of this section, one or more 120-volt, 20-ampere branch circuit shall be provided to supply bathroom(s) receptacle outlet(s) required by 210.52(D) and any countertop and similar work surface receptacle outlets. Such circuits shall have no other outlets.

Exception: Where the 20-ampere circuit supplies a single bathroom, outlets for other equipment within the same bathroom shall be permitted to be supplied in accordance with 210.23(A)(1) and (A)(2).

Thank you for the advise! It looks like I’m going to have to run a separate 15amp circuit for the lights.

Thank you for the insight and deep dive! You are definitely right as I have everything on a 20amp circuit, but I have done it to code with it being GFCI. I had everything back to studs, but it’s dry walled now. I think I will just bite the bullet and run a 14/2 as suggested and separate the lights out.

that’s not the wording of the code. Exception: Where the 20-ampere circuit supplies a single bathroom, outlets for other equipment within the same bathroom shall be permitted to be supplied in accordance with 210.23(A)(1) and (A)(2).

OUTLETS can be shared, OUTLETS is the key word you changed.

>OUTLETS is the key word you changed.

Good lord. I didn't change anything. YOU are misunderstanding what the definition of 'outlet' is in the NEC.

NEC Article 100 defines an “Outlet” as a point on the wiring system at which current is taken to supply utilization equipment. The term “outlet” is often misused to refer to receptacles. Receptacle outlets are only one type of outlet. Other types of outlets include lighting outlets, appliance outlets, smoke alarm outlets, equipment outlets and so on.

A lighting outlet is an outlet intended for the direct connection of a lampholder, a luminaire or a pendant cord terminating in a lampholder. Luminaires can be ceiling-, wall- or even floor-mounted (See Figure 2). While the wiring for lighting outlets can terminate in outlet boxes and in other boxes under certain conditions, it can also terminate in junction boxes of luminaires.

So, in short, just because you don't understand what 'outlet' means in the context of the NEC, doesn't make what I wrote wrong. If you're still adamant, go ask someone in /r/electricians. I've been through hundreds of inspections, have you?