The movement component bases its result on findfloor - if the preview window has nothing as a floor and the character is currently simulating, it makes sense it wants to fall.
Normally, persona (in 4.x at least) does not simulate the character movement, the idle plays, whatever you toggle the value of plays, but since there is no sim, the movement component is not in a state to return values.
Because you are making it return values anyway, then no floor = fall…
As a side note:
Not to knock on your interface idea, but once again le’s remind everyone that having any code in the animation blueprint is wrong.