I'm using Photo 1.10.5 on a macbook pro running Monterey OS.
I created a new document and placed 3 separate files into it so they're embedded. I opened and edited these on the fly.
When I try to save the document I get "Save failed because the file could not be written to".
I've tried save as, saving to different locations such as desktop but no luck. A file is actually created when I try to save but if I try to open it I get "file type not supported". I've also tried copy-pasting the embedded files into a new document but I get the same problem when I try to save this.
And as I'm writing this.... Affinity Photo quit unexpectedly! I reopened, recovered the document, it opened with a few last changes missing and.... I was able to save it!
So, problem solved for now but I had to wait for it to crash to fix it!