On the "Monitor" screen, in the area where you can control the extruders, leave extruder 1 selected.
Scenario 1: Now select a model to print and specify to print with extruder 2 only; it never works - it will always switch to extruder 1 when it starts to print.
Scenario 2: Now select a model that prints with both extruders and that starts printing with extruder 1 - let the print run until it switches to extruder 2 and starts printing, NOW switch to the “Prepare” screen then back to the “Monitor” screen while it's printing with extruder 2, once the gcode buffer runs out it will flip back to extruder 1 wrongly!
It seems that when switching to the “Monitor” tab it issues a T# command that then screws up anything printing - or could depending on which extruder is printing and which extruder was left selected in the area where you can control the extruders in the “Monitor” tab.
It seems an (easy?) fix is when switching to the monitor tab do NOT automatically send a T# command - just send the T# command when someone selects an action like Extrude or Retract right before doing the action. Although it's not just the act of switching screens - see first scenario with extruder 2 only.