This morning I worked with a colleague to see what is going on.
There is a step in the logic of the Multi where it will not raise the charge voltage (for the solar chargers) while it is still charging from AC-in.
In a system with a PV-inverter on the input side, like yours, it will be importing that energy and charging the battery. So the system is charging from AC-in.
Once the battery gets full enough, so that it stops accepting charge current, the Multi will then raise the charge voltage. The effect is that the DC-coupled solar charger can stop charging for a period of time while the system charges with only the AC-coupled PV.
Once the battery is fully charged, the situation will usually resolve. The larger the battery, the longer that will take.
It also appears to depend on minute calibration differences (beyond what we can synchronise), which makes the solar charger back off at the charge voltage, while the Multi continues to charge from AC.
Once you get past this hurdle, the system will feed DC-coupled PV into the grid for the rest of the day, unless a large load or loss of production causes the charge state to return to bulk.
So we understand now why this happens, but I don’t yet know what to do about that.
What you can do right now: You could move (some of) the AC-coupled PV to AC-out. Or you can move more of the DC-coupled PV to AC-coupled, as you suggested.
These are workarounds, but I didn’t want to leave you hanging while we work this out.