On my Mac (Sequoia 15.5), when launching Syncthing, with my URL configured to 0.0.0.0 (instead of 127.0.0.1) the flag was red next to the API in the toolbar icon.
When I went to preferences it had the URL as https://0.0.0.0:8384 and “Test” was red. When I changed it to https://127.0.0.1:8384 and hit “Test” it went green, but that URL didn’t stick when I quit and restarted.
In the end I browsed the code and found the default used to read the URL, and running this command from the terminal fixed things for me:
defaults write com.github.xor-gate.syncthing-macosx URI "https://127.0.0.1:8384"
Posting this in case it is useful to anyone else.