If you open an older-version project in ARCHICAD 17 or later, the Legacy Mode for element intersections will be turned on by default to maintain the appearance of your model.
In Legacy Mode, the automatic and Building Material priority-based intersections implemented as of ARCHICAD 17 will not take effect in 3D. Connection methods used in ARCHICAD 16 and earlier versions will be used. You may prefer this when working on an older project, to ensure that your existing intersections remain intact.
Legacy Mode is controlled in Options > Project Preferences > Legacy.