I installed the OTA 8.1.0 update on my Nexus 5X last night, and after the reboot Syncthing is showing as “disabled”. It worked perfectly on 8.0.0 with “always run In background”, “run only on wifi”, and “restrict to certain networks” run conditions. I have tried unchecking the restrictions, leaving only “always run in background” checked, and even tried unchecking that and running manually but it is still disabled.
Just read another thread where the OP posted logs. It turns out 8.1 removed permissions to SSID (location), and that this explicit permission will need to be requested in a future version of Syncthing.
I tested this theory successfully by removing the SSID restrictions from my run conditions, and Syncthing is enabled now.