EPUB to PDF Doesn't Meet Latest Kindle Scribe Requirements?

Hello, all, and thanks, in advance, for the help.

Before today, I very regularly converted my epubs into pdfs so I can write on them and highlight passages on my Kindle Scribe; however, when I tried to do so today, Amazon sent me a message that my pdf no longer meets their requirements and I won't be able to use the write-on or highlight tools.

I used the "Send-to-Kindle" via email method and doubled-checked that the issue persists even when using the old-school hardwire method.

Any ideas?

Thanks for everything!