This happens on my AR300M also. Has been for a long time. When it fails i must change the local ip of the WireGuard Server, activate it , deactivate it, change the IP once again and then it works. I have to do this often and it is just irregular. I have the latest GLI firmware 3.104 / 18.06 as of NOV 2020 but realise this is way behind the current OpenWRT firmware which is 19.07.4.