Amazon's web page for the
Scribe states, "Scribe does not support handwritten sticky notes on manga, comics, graphic novels, magazines, or newspapers, though they are still available for download and look beautiful on the 10.2” 300 ppi display." The
Fix Issues page I linked previously advises, "Try sending the document again through Send to Kindle to make sure it has the latest features enabled for Kindle Scribe and Send to Kindle."
This make me wonder whether Amazon did not bother to implement sticky notes for MOBI format. That would explain the lack of support for periodicals and a potential need to resend documents to have them converted to the correct format.