Due to human error plus a confusion on the build server, I uploaded both the
syncthing-macosx-amd64-v0.14.39.tar.gz (correct release) and the
syncthing-macosx-amd64-v0.14.39+1-gf0ccc1e6.tar.gz (current master, not a signed release) archives.
Syncthing upgrade process picked the wrong one (lexically first in the list), and correctly noted that it was not a properly signed release.
I’ve removed the incorrect archives from the release, Mac upgrades now work. Thanks for noticing!