Just for the record, I have a feeling that this issue may actually have nothing to do with connectivity, but is rather caused by https://forum.syncthing.net/t/syncthing-stuck-gui-seemingly-responsive-but-nothing-is-happening/16828. The GUI on one side reports the other as disconnected, while the other one says otherwise, but only because the GUI has been stalled due to GC. Once GC has finished, everything unblocks, and the devices reconnect.
The devices discussed here are the same as the ones listed in the other topic.