After I run a firmware update on my 10G switch, or simply reboot it, OPNsense gateway monitoring starts to fail frequently. I can reach the OPNsense box fine on LAN (ixl0, native) but the following problems seem to be responsible for gateway monitoring problems. Restarting the VM (passed through the Intel(R) Ethernet Controller X710 for 10GbE SFP+ controller with Proxmox) immediately fixes the problem.
Sep 5 11:18:17 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl1 type=LINK_DOWN'
Sep 5 11:18:19 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl0 type=LINK_DOWN'
Sep 5 11:18:19 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl0_vlan200 type=LINK_DOWN'
Sep 5 11:18:20 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl0_vlan106 type=LINK_DOWN'
Sep 5 11:18:22 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl1 type=LINK_UP'
Sep 5 11:20:08 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl0 type=LINK_UP'
Sep 5 11:20:41 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl0_vlan200 type=LINK_UP'
Sep 5 11:20:59 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl0_vlan106 type=LINK_UP'
Sep 5 11:21:21 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl1 type=LINK_DOWN'
Sep 5 11:21:22 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl0 type=LINK_DOWN'
Sep 5 11:21:22 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl0_vlan200 type=LINK_DOWN'
Sep 5 11:21:22 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl0_vlan106 type=LINK_DOWN'
Sep 5 11:21:23 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl1 type=LINK_UP'
Sep 5 11:22:22 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl0 type=LINK_UP'
Sep 5 11:22:22 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl0_vlan200 type=LINK_UP'
Sep 5 11:22:22 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl0_vlan106 type=LINK_UP'
Sep 5 11:22:50 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl1 type=LINK_DOWN'
Sep 5 11:22:51 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl0 type=LINK_DOWN'
Sep 5 11:22:51 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl0_vlan200 type=LINK_DOWN'
Sep 5 11:22:51 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl0_vlan106 type=LINK_DOWN'
Sep 5 11:22:52 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl1 type=LINK_UP'
Sep 5 11:23:56 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl0 type=LINK_UP'
Sep 5 11:23:57 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl0_vlan200 type=LINK_UP'
Sep 5 11:23:57 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl0_vlan106 type=LINK_UP'
Sep 5 11:24:25 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl1 type=LINK_DOWN'
Sep 5 11:24:26 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl0 type=LINK_DOWN'
Sep 5 11:24:26 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl0_vlan106 type=LINK_DOWN'
Sep 5 11:24:27 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl1 type=LINK_UP'
Sep 5 11:25:14 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl0 type=LINK_UP'
Sep 5 11:25:14 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl0_vlan200 type=LINK_UP'
Sep 5 11:25:14 OPNsense.local-lan devd[93739]: Processing event '!system=IFNET subsystem=ixl0_vlan106 type=LINK_UP'