Thanks PaulEC, that does indeed seem like a lot of work, with no apparent guarantee it would happen again. Although I despise Adobe and their subscription model, I never had this issue with Photoshop. Saving was easy-peasy. I have no answers on what exactly caused this, so I can avoid it happening in the future, and it doesn't appear that the techs at Affinity know what's causing it either, which doesn't inspire a lot of confidence. 😒😖
It looks like what I'll have to do is save a version of each step of my project at every single stage of development: Complete one section, save as a TIFF file. Work on the next section, save as a TIFF file, and so on. That way, if this happens again I can go back to the most recently completed version. I just wish Affinity would get it together and find a fix for this. I'm not the only person this is happening to, apparently. 🤬
Just a shot in the dark, but do you know of any way to retrieve the work from a truncated file? As I said, SOMEthing is taking up 84.8MB of space in this file. There is data there.