Hello Frank. Thanks for this new information.
I would like to repeat some advice that Matthias provided (on May 5). “If you have connected the MPPTs to the starter battery they have to run in stand-alone.”
Quite simply, for devices connected in a data network to behave sensibly they need to be connected within the same electrical circuit. And vice-versa, devices that are not connected within the same electrical circuit should not be connected in the same data network. Matthias provided other clues; when the house battery is full, the Smart BMS 12/200 is cutting the electrical connection between the MPPTs and the Smart Shunt, although the network connection remains in place. The MPPTs learn from the Smart Shunt that the voltage it is sensing has dropped, so they pump out more and more, which you see at the engine start battery.
If your purpose is to use the solar panels and MPPTs to keep the engine start battery charged, use them in a stand-alone manner. Delete the MPPTs from the Smart Network.
It’s not clear from your comments whether you are wanting to use the MPPTs to charge the house battery as well (say, when the engine is off and shore power is disconnected). This would be desirable, so perhaps this is what you are trying to do? After you have deleted the MPPTs from the Smart Network, I expect you will find that they will still be able to contribute to charging the house battery, via the cables running from the engine start battery/alternator to the Smart BMS 12/200. If the house battery is not full, the BMS will allow charging through its alternator connection. The MPPTs will be sending current there. What do you see?
As I mentioned earlier, I think the best solution is to connect the MPPTs to the bus-bars for the house battery and not to the engine start battery (and include the MPPTs in the Smart Network). This is shown in the diagram Matthias provided. Then, add a battery to battery charger to keep the engine start battery topped-up while at anchor. 