Photo can't open its own .tif files

[tjkp]

I've just been working on a couple drum film scans, adding layers for color correction and general toning, and saved the files as .tif with Affinity layers. The files are huge, from 800 MB to over a GB each depending on the image, but Photo seemed to handle them just fine from opening to cropping and adding multiple layers. I closed the files after saving and upon trying to reopen them I get the error message (screenshot attached) that the file type is not supported. Just updated Photo, closed and reopened the application, tried opening the file via double-click in Finder and from the File > Open dialogue, the software hangs for a moment with the message "loading 1 document", and then it's the same error every time. What's going on here?

Basic info: Mac 10.15.7; Photo 1.9.0; files located on a raid directly attached to my MacBook Pro; I'm not inclined to post the files publicly, and not sure I could given their size