Hej,
this happens very often and everytime I lose my work. I work on a document in Designer on my 2020 iPad Pro 12.9. All things are uptodate. If I switch to other apps, eg. Lightroom, because I export one artboard to PNG to prepare for use on my website, iOS decides to cleanup memory an ends Designer. But my work isn't saved. Why is there no automatically saving? I don't understand why this isn't implemented. I don't observe this behavior in other apps than the Affinity apps. In Photo this happens on a similar way. Out of my point of view this is the most critical behavior in your software and needs a fix as soon as possible.
This is the only thing what I see that doesn't work properly. Ah...and save by hand isn't a valid workaround. Sometimes you forget it if you are in your workflow and sometimes I switch between 3 or 4 apps for an illustration.
BR
Thorsten