After checking every part and the xml file I can say that all tanks I'm aware of are filled with monopropellant but the flight view shows that they're at 88% capacity.
To make this weirder activate the first stage, it just moves a pair of pistons in the tower but mono suddenly drops to 33% with no apparent reasons. The connections don't change with the activation of the stage, so there should be no change in fuel percentage
Dev Notes: If you launch a craft with an empty mono nose cone, it can confuse the mono fuel level.