If it comes from a third party repository, please indicate that when asking for help - that’s very important information.
Given you are using the CoreELEC repository, i.e. their builds, the update is likely unintended. Probably changes something about their build process and somehow messed up disabling auto-upgrades. In any case, that’s a question for the support venue of CoreELEC.
As to the logs you posted: They only show how syncthing stops, nothing more. Logs of the subsequent start attempt would be interesting. If there aren’t any, you need system logs of whatever manages running Syncthing. That’s a gain a question for CoreELEC.
Yes, I have realized that as well - it’s all the matter of CoreELEC repository.
For the old version 9.2.7 apparently repository contain old Syncthing version and when autoupdate is performed through Syncthing itself it is causing this critical issue.
For the new version 19.1 Matrix there is available Syncthing 1.15.1 in CoreELEC respository.
I have already asked the question if we can expect Syncthing update in CoreELEC 9.2.7.