I found this a long time after it actually happened, and long after whatever logs from the actual event were gone. When I found it, all nodes claimed they were out of sync, and all of them were complaining that they wanted to sync the folder "Backgrounds" but that it didn't exist on any online node. None of the machines had a folder named "Backgrounds" or "backgrounds" anymore.

I imagine what happened is that this was somehow related to the fact that the PC and Mac have case-insensitive file systems, and the Linux machine does not. Somehow one of these machines renamed the folder (I have no idea how this could have happened). I would hazard to guess that SyncThing on one of the nodes (probably Linux) decided it needed to delete the "backgrounds" folder because it no longer existed, and then sync the new "Backgrounds" folder. If deleting the "backgrounds" folder was synchronized back to the PC or Mac though, they would have also deleted the "Backgrounds" folder, since in case-insensitive-land, they're the same folder.

I went digging through their github at the time to see if there was an issue about case sensitivity and didn't find anything that was relevant and open. I didn't have a lot to go on, and my attempts to recreate it failed at the time, so I didn't bother opening an issue. It's entirely possible my specific issue is fixed by now.