Once you change a body's behavior from flexible to rigid, it no longer needs to mesh the whole body. The geometry and density completely defines the mass and moment of inertia.

ANSYS does not do a good job animating the rigid bodies. Ignore them.

You said you wanted the model to deliver 1.88 Hz.  I gave you two methods to achieve that. 1) reduce beam thickness 2) add mass.  Choose one.