CISA Website re: Ballot Safeguards. It is DEFINITELY POSSIBLE that select jurisdictions have some sort of secret watermark or security device on ballots. Maybe not nationwide, but perhaps a couple key swing states?

Reality: Safeguards are in place to prevent home-printed or photocopied mail-in ballots from being counted.

Rumor: A malicious actor can easily defraud an election by printing and sending in extra mail-in ballots.

Get the Facts: This is false. Committing fraud through photocopied or home-printed ballots would be highly difficult to do successfully. This is because each local election office has security measures in place to detect such malicious activity. While the specific measures vary, in accordance with state and local election laws and practices, such security measures include signature matching, information checks, barcodes, watermarks, and precise paper weights.

https://www.cisa.gov/rumorcontrol