NetFlow Traffic Diagram WAN Interface monitoring crashes

[Smash]

Hello there, I have searched the web for hours for the solution of a very strange issue I'm facing, to no avail.
Have a look of the following time-lapse of the Traffic Diagrams during three consecutive online speedtest runs:

During the first run, everything is normal (WAN In equals LAN Out), on the second run, the WAN Diagram line suddenly drops to 0 and stays there during the third run (only LAN Out shows a peak).
The only way to get the WAN Diagram line to show something else than 0 again, is to disable the VirtualIP it's connected to and enable it again. Then it works for a few seconds until it crashes again.
The issue persists on both of my firewalls which run the exact same configuration.

Setup: OPNsense 26.1.5 running as a VM on Proxmox 9.1.6 being connected by 3 virtual VirtIO NICs.