Mail-in ballots can't be officially counted until 7am (or later) when polls open, so even though the courthouse received your ballot last month, it wasn't opened until today. I can't tell you why they would've cancelled it, but unfortunately there's no way to find out sooner than election day if your ballot was officially counted.

Not sure what county you're in, but here in Northampton County, we had about 35,000 mail-in ballots and only like 25 people to sort through them all, so unfortunately that means some ballots weren't getting examined until 6 or even 7pm which left very little time for voters to run to their polling places (if they even found out there was an issue).