WAN Speedtest drops 90% after exactly 5 minutes of saving interfaces.

[wbg]

I have a Deciso DEC850 running OPNsense 22.1.6-amd64.
When the OPNsense is connected to a fiber modem/router I get full speeds continuously.
When connected to a fiber mediaconverter with a VLAN 100 tag on the WAN it drops after exactly 5 minutes to about 100-115 Mbps.

Very easy to reproduce, it takes 5 minutes everytime.
Can anyone tell me if there is anything specific happening 5 minutes after saving an interface?
Saving and applying an interface will somehow reset it and it has full speed again 946/947 for the whole of 5 minutes.

Traffic through my VPN does not get limited.
I have no Shaper active.
I have assigned and enabled the physical interface that the VLAN is tagged on.

[wbg]

This just makes no sense.
Unchecking the disable gateway monitor just plain fixed it.

[opnfwb]

After 5 minutes when the speed drops, is the gateway showing packet loss? You may need to specify an external gateway monitor IP address. Sometimes it also helps to set the Data Length payload for the pings to a small number, I usually to 3 to 5 bytes. Some ISPs don't like 0 byte ICMP.