HA Active-Active issues with software-switch ports

[AnotherFortiUser]

Hi, we have two FortiGateRugged 60F in an active-active HA cluster.
internal1 and internal2 ports were originally members of a hardware switch interface, with 5 VLANs.

We changed the configuration to a software switch interface, to have a more granular control over the vlans for the seperate two ports, because the first approach doesn't allow to assign some VLANs on internal1, and some other (but overlapping) VLANs on internal2.

The problem: Prior to the change from hardware switch to software switch, we could unplug the cable on port internal2 on FortiGate A (Primary, higher priority) and the traffic would be forwarded through internal2 on FortiGate B.
But this isnt possible anymore. If we unplug the cable, there is no rerouting/forwarding and the connection is simply lost. I guess the FortiGate cluster still keeps FortiGate A as Primary, even though one interface is down?

I wanted to add the two interfaces to the "monitored interfaces" of the HA, but this isnt possible - apparently due to using the software switch type. But in the earlier setup with the hardware switch, there weren't any interfaces monitored as well...? I confirmed this by looking in the older configuration file.

How can this be possible? Does traffic of a down interface get forwarded to the secondary FortiGate without the FortiGates in an HA changing their primary/secondary status?
Is this even possible with a software switch? When I searched online, I found multiple sources stating that software switch / VLANs interfaces cannot be monitored..

Thank you in advance!