My first post to this topic: QGIS 3D - no terrain altitude binding There I have had problems with 3D objects from a point layer. I solved this.
Now I have a problem with 3D objects from a polygon layer. I use polygons / buildings from osm. All buildings with the same high. Within qgis2threejs is all ok. But with QGIS 3D not: No altitude binding. But I need for my ideas 3D of QGIS, I can not use qgis2threejs. Any ideas to solve the problem? I need the same result as for the buildings within qgis2threejs.
I get only a good result if I use flat terrain.
above: The buildings with the black lines are examples, not the only wrong building-heigh-positions. The buildings left are OK, on the right wrong.
Windows 10, QGIS 3.10.6
edit: try on QGIS 3.12.3: only crashes....oh....