If you're using the Qgis2threejs plugin to produce 3D maps on QGIS, there exists an interesting change between older versions of QGIS and the newer 3.0 ones.
The floating, clipping map layers that refuse to lie correctly on a DEM.
The solution: Change the QGIS project CRS to EPSG 3857 (Pseudo Mercator). Since QGIS has permanently enabled on the fly CRS transformation on 3.0 and newer versions, the Qgis2threejs plugin requires you to specify EPSG 3857 to work correctly.
- Jasper Stone