Need more information.
The slot positions are somewhat of a misnomer when it comes to Autosaves; I would need to see the times listed on the slots to try and make sense of what is happening, but essentially the system also saves the real-world time each autosave was created at, and uses that to order the list. It will then pick a slot to save into based on which slot has the oldest real-world time listed.
It's possible something might have gotten goofed up as part of the Electron serialization changes, but it would have to be really specific to the state somebody had their autosaves in at the time the import was done. My rough guess is maybe its trying to reimport the slots again and again, like they'd deleted a bunch of the slots (which only deletes the NEW files, not the OLD IndexedDB data), and that old data somehow gets reimported... but that wouldn't explain why the real-world time was being fucky.