Please keep us informed! An interesting puzzle for sure.
Not clear on why a boost button at all when the control centre does that anyway. Also you have a Trimetric with shunt on the engine batt plus the Victron reading the engine batt's voltage but no shunt, which is on the house batts?
Any way there can be confusion with the two monitors on the engine batt that might light up the red lamp when solar is on but not when solar is off?
Too many moving parts for my tiny mind! :) I would still worry whether the engine batt is really getting charged with that light on. I would want a second opinion (multimeter) besides whatever the Victron is saying via your phone.
Best of luck solving the puzzle!