I am on Tumbleweed and use latest version:
v1.23.7, Linux (64-bit Intel/AMD)
Maxim 8356 0.0 0.0 732572 17916 ? Ssl 01:48 0:00 /usr/bin/syncthing serve --no-browser --no-restart --logflags=0
Maxim 8365 19.5 0.4 1077380 158688 ? SNl 01:48 2:14 /usr/bin/syncthing serve --no-browser --no-restart --logflags=0
In systemd I used UMask to set it and it is applied, we can check it by:
grep '^Umask:' "/proc/8356/status"
Umask: 0077
grep '^Umask:' "/proc/8365/status"
Umask: 0077
But when Syncthing creates files it ignores umask:
-rw-r--r-- 1 Maxim Maxim 8 Aug 22 16:14 test.txt