Is there any particular reason why you’ve set both sides to “Send Only” ? Sync is never going to work this way, you need at least one side to be able to accept changes.
Have you tried to simply pausing the folder instead? I’ve got a feeling that two sides set to Send Only are simply not going to play nicely together (even if you do the whole switching thing), leading to potentially untested consequences.
I did try that before, but didn’t realize that that was the intent of pausing a folder. Either way, the issue persisted, even while paused/after unpausing.
At the moment I’d recommend removing the folder from Syncthing on one device completely, then add it again and see what happens once all scanning and syncing has finished. Make sure that at least one side is set to Send & Receive.
Also, please make sure that “Ignore Permissions” is enabled in the Advanced tab in the folder settings on both devices.
I’d hope that bumping a topic isn’t against the forum rules.
My laptop is currently out for repair, and has been for the past week and a half, so I haven’t been able to do this yet. They’ve assured me that my drive hasn’t failed in the process and that they haven’t opened any apps on my machine that weren’t set to auto-start before I gave it to them, so nothing should have erroneously synced while it’s been gone.