A new undocumented feature in ArchiCAD 17 is the ability of beams to heal/mitre with each other in a z-axis change of angle (we don't know its undocumented!!).

Beams have always mitred in plan (i.e. X or Y axis direction change) but this new functionality drastically reduces modelling time in some very specific features (such a s roof treatments).

Users must be aware that only a Z axis change is blended correctly, a combined Y+Z or combined X+Z change will not heal correctly.

See below screenshots from ArchiCAD 16 and ArchiCAD 17, showing the same geometry but different mitred results.

In ArchiCAD 17, for this functionality to work, please ensure the option for 'legacy intersection method' is turned off/unchecked (shown below, and found in Options > Project Preferences > Construction Elements)