I don't use those windows so I am not all that familiar with them but I just had a look.
It seems if you turn on the external sill it does not cut off the bottom external casing.
However when you turn on the internal board, it does cut off the internal bottom casing.
I have no idea why.
I don't see any way to overcome this.
All I can suggest is to not have both the internal casing and the board on at the same time.
Have just one and model the other separately (easiest to just model the board with a slab).
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11