This is the default way Revit joins two cavity walls with an extended brick layer at the base.

This is what happens when switching to Squared Off.

Miter works but it screws up the join between any other walls!

Whatever methodology the developers came up with for controlling how walls join needs to be scrapped.
It just doesn't work. Two things need to be done here.
1. If one wall joins two different walls at two different heights EACH join should be resolved independently of the other so they don't influence each other in bad ways
and
2. fix whatever crappy programming can't handle the simple joining of two multilayer walls in the first place.