moving inner boundary by defining prescribed displacement

Hello everyone

This may have an obvious answer but I am a bit new with Comsol so I am playing around with various interfaces, particulary the ale interface. I am trying to deform an inner boundary by prescribing the displacement. I attached an axisymmetric 2D plot of my model. I set the inner boundary to a 2mm z-displacement and 0 along the x-axis while all the other boundaries were fixed. I formed a union with my geometry so I am assuming that I do not need to explicitly define displacement for the adjacent boundaries since they will move in accordance with the moving boundary.

I am confused why the moving boundary resulted to a trapezoidal shape rather than a flat surface that moved 2mm up. Did I set up my BCs wrong?

Btw, the cylinder that contains the boundary I am interested in represents a conductive fluid and I am trying to deform it using Lorentz force. The line element is the coil (just in case someone wonders why I have this geometry). But I disabled the mf interface just to understand how the moving mesh interface works.

Thanks!