Bitcoin Core drops incoming connections and doesn't recover

[RonSo]

App: Bitcoin Core v19.0.1 64bit
Windows 10
Internet: PureVPN with fixed IP, Port forwarding
Router: MOFI 4500-4GXeLTE V2

Relevant Parameters:
Listen=1
maxconnections=25
server=1

Problem: Incoming connection fails Global Bitcoin Nodes "Check Node" test randomly. My full-node can be up for as long as 48 hrs without failing, but, as short as 10 minutes before failing. Once it fails the Check Node, I have to shut the node down and restart it in order to pass the Check Node test again. The minimum of eight outgoing connections always seem to be there.

Any thoughts as to why this happens?

[HCP]

Is it just the "Bitnodes" test that is failing... Or are you also seeing zero "incoming" connections on the node as well?

If the former, probably just an issue with Bitnodes... If the latter then possibly some weird glitch with your router or DNS or possibly even the VPN blocking traffic...