Archicad used to mitre corners but has not done so since version 17 from memory.
Each skin of your wall will trim according to the Building Material strength - stronger materials will cut away the weaker ones.
Where the BM is exactly the same strength (i.e. same material), Archicad will decide.
You can override this with the 'Junction Order' of the walls.
If you really want to (but I don't recommend it), you can go into the project preferences and turn the Legacy option on for intersection and surface methods - but this will affect other aspects of your model - read the info button for more details on what will be affected.
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