Logs from the affected syncthing clients may be more helpful than the relay debug log.
From those logs alone, it looks like neither of the two devices actually join the relay. They both attempt to dial each other, but when neither is persistently connected to the relay, this can’t work.
Perhaps more clear: In order for a device to be available via a specific relay, it must be configured in Actions -> Settings -> Connections -> Sync Protocol Listen Addresses.