For me it is not recording credentials with the assumption I would simply remember them later, while having every opportunity to archive them before eventually forgetting. Also, not keeping detailed enough notes & photos of exactly how my hardware is attached.
My theory for what happened is:
There might have been some delayed writes on Arch, but that’s no the main issue.
When booting into Fedora and running an update, the state of the filesystem changed to the point that when resuming Arch, it put the filesystem into an extremely inconsistent state (where the Arch system might have cached (meta)data that was changed since hibernation).
Also, to clarify, I still managed to recover my data, but the FS was not mountable and btrfsck couldn’t do shit. And I’m still using that Arch install to this day. XDDD