I test on ARM64, Syncthing 1.28.1 with option --upgrade-check
I see the option in on-line help https://docs.syncthing.net/users/config.html
And in command line help sycnthing --help
This line was inside a .sh file to startup syncthing.
When I add --upgrade-check it not startup, if I revome it startup.
If call in sheel i see that when add --upgrade-check, syncthing prompt me the help.
--upgrade-check checks for an upgrade and exits. That does not seem to be what you want to do given all the other options and launching into the background.
The test with sheel, I hade made some day ago, and it reply me with help.
Now I re-test and I see it function, I not know what I do some day ago.
I understand the problem, with --upgrade-check syncthing check for update and terminate, but I had supposed that function as same as --no-upgrade that force and ignore the value configured in config.xml