Let me clarify:
I had an app that relied on scanning for bluetooth devices in the area, that hasn’t been working for years on my Mate 10 Pro. Just recently I found that activating “Access my location” (Settings->Location->“Access my location”) got this other app to work, and I think that can be explained by @tomasz86 post, as I believe it applies to bluetooth just as much as to wifi (although, my country’s “Corona” app worked flawlessly, which I suppose works on a similar principle)
This setting is not a “per app” setting, but has to be enabled globally. I am not sure if it helps you with your issue, but I know that getting that phone to autostart or “not kill” (or not delete) something can be a pain.
I do not autostart syncthing on that phone, as I use it only “on demand”, so I can’t really comment on that. But:
You may want to check the battery settings on the preinstalled “optimizer” app. You want to set syncthing to “Manage Manually”, I believe, otherwise EMUI will kill it.