I am experiencing the same behavior.
ProtonVPN to allow LAN & GOS "Block connections without VPN" active
= Connection to LAN possible with syncthing
I believe I recall this not being the case before the recent ProtonVPN update...!?
I recall being annoyed having to deactivate "Block connections without VPN" for syncthing to connect.
Going into the VPN settings just now.. I realise that the "Block connections without VPN" toggle is IN the ProtonVPN VPN Profile...
Does this mean ProtonVPN to decides how the toggle works...?!