OpenVPN Tap Mode - ARP incomplete - sometimes

[alhh]

Hi all,

I have a strange behavior/problem.

I set up an OpenVPN Server, TAP Mode for Windows Gaming.
It’s working fine, all friends can play reliable all LAN games, use file-shares and ping each other (if their clients allow it).
So far, so good.

While the VPN clients are connected and actually playing I see changes in the ARP table of my Netgate SG-3100.
At the beginning each client as an entry on the OpenVPN-Server-Interface and one on the bridged Windows-Interface.
Both entries may disappear during the established and working (aka gaming, file-shares, etc) connection.
And at the moment the ARP entry is lost (aka becoming incomplete) on the (bridged) Windows-Interface, I am no longer able to ping the clients from the Netgate. What makes sense.

What I do not understand is, why the ARP entry becomes incomplete, while the connection is used AND the Negate pings all clients regularly.

To make it even more strange (imho) the state of the ARP entries may change while the clients are connected.

Do you have any idea, tip or pointer for that strange behavior?

Best regards,
Andreas