Hello,
I have difficulty that stops my work.
I need to extrude bottom part for 2 more milimeters on the object attached.
Attached is original and extruded to see what has happened.

I tried to select the bottom side and extrude.
Also tied to make another object from copied bottom plane and do a boolean union with that object (also tried with 1mm overlap).

What happens is that every time I try one or more corner (filleted) walls will dissappear.

Also tried to mirror those walls from those that were left and join everything (also union) but it's clear it will not join.
I select the main objects (it says solid in description) but copied walls remain unselected.

I also tried with MoI v4 but it does almost same thing.

Any idea how to properly extrude this without artifacts?

Thanks a lot