Why this happened
It is almost always one of two things
Nothing is wrong with your registration, and you are not in any trouble. Leon County compares the signature on your return envelope to the one in your voter record, and a ballot gets set aside when that comparison fails.1 There are only two ways it fails.
Reason 1
The envelope was never signed
Florida counts a mail ballot only if the Voter’s Certificate on the outside of the return envelope is signed. It is easy to seal the envelope and drop it in the mail without signing the back — and once it arrives unsigned, the county cannot open it.
Reason 2
The signature did not match your registration
Elections staff compare the signature on your envelope to the one in your voter record. If your signature has changed since you registered — which happens to nearly everyone over time — it gets flagged. This is not an accusation of anything. It is a routine mismatch with a routine fix.
Either way, the fix is the same. You sign one form called a Cure Affidavit, attach a copy of your photo ID, and send both to the Supervisor of Elections. That is the entire process. Your ballot is then counted normally, exactly as you filled it out — nobody sees how you voted, and nobody re-opens the choices you made.