I can verify your issue with a TIFF, but not with a PDF in 10.11.5. The PDF annotations remain editable after saving and re-opening.
I use annotations in Preview extensively, but not usually with TIFFs, so I can't speak to when it may have changed. I can say that PDFs are behaving now as they always have.
The "Markups" feature that some of the reps were addressing is in Apple Mail. You can now annotate some attachments directly in Mail.
By the way, I really enjoyed your discussion with the senior advisor of expected behavior vs. bug.
On a Mac since 1984.
Currently: 24" M1 iMac, M2 Pro Mac mini with 27" BenQ monitor, M2 Macbook Air, MacOS 14.x; iPhones, iPods (yes, still) and iPads.
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