Monopropellant drains from the last stage then gradually to the first.
This means that when a staging occurs, fully topped monopropellant tanks will be thrown away.
What should happen is that the lowest (first stages) tanks should be drained first, so that when it stages the empty tank will be thrown away while the remaining tanks on board stay fully fueled.
Here is a craft with three stages. Use translation mode and pitch down to drain the tanks with the single RCS nozzle on the top, and notice it drains the topmost tanks first.
There is also a bug report