This happened on a new file, after attempting to copy-paste a part of the sprite. I got a message telling me some other program was already using the copy-paste function, and suddenly the text was unreadable.
The text that was still showing up correctly was ruined after hovering.
Hi @CrispyPear, I would recommend you to update your version of Aseprite to the latest one. Anyway I suspect that your installed sprite sheet font file is broken.