Okay, let's roll up our sleeves with this one:
Danny1234 transferred my photos through USB into the camera folder in DCIM (screenshots in the screenshots folder in DCIM) earlier today before shutting down my phone.
How exactly did you transfer these files? From Windows, Mac, Linux?
Danny1234 uploaded the photos to Fossify Gallery
Fossify Gallery is not a cloud service. You can't upload anything onto it. It's just an image viewer for files you have in your phones storage.
Danny1234 I checked the Fossify recycle bin and they are gone.
Photos only appear in the recycle bin if they were deleted inside the app.
Danny1234 I don't understand how my photos can just be deleted without any notice or action on my end.
Me neither.
Danny1234 I checked the Samsung (my only backup in this case) which I clearly specified I wanted to COPY (not move) from (while using the Pixel to access the storage of the samsung).
I am now confused, did you transfer your whole library from another phone? If the Samsung was your only backup, where are the original files stored?
Danny1234 All my pictures are gone there too, and since I don't use a Google account there is no cloud backup.
Sorry if this sounds rude, but the general rule is: if data doesn't exist in 3 location simultaneously, it doesn't exist at all. Important files should be backed up on multiple mediums with 1 offsite backup (store in a relatives house for example).
Danny1234 I apologize if this is turning into a rant but for a first time user of GrapheneOS this is a truly awful first experience.
I am not sure if the OS is to blame here. This can have a dozen causes not related to the OS at all.
Danny1234 My pictures appeared to be replaced with my screenshots.
This is a very unclear statement. So some photos made it through while others didn't? This, again, requires more information about how the transfer was performed.
Danny1234 Why would Fossify Gallery do this?
Some users seem to have strong opinions on the stability and security of Fossify apps. It appears like the whole project is maintained by only one person, so I'd be cautious with their apps.