The files: assimpsceneimport.dll/pdb (and assimpsceneimportd.dll/pdb) are missing from Qt6 binaires.
"C:\Qt6\6.1.0\msvc2019_64\plugins\sceneparsers" only contains the gltf importer/exporter.
These files are provided in the MINGW binaries but not in the MSVC binaries.
The same story already appears in the past:
But it was fixed and working again in 5.15.0 (I have not tested all versions):
But the files are missing again in Qt 5.15.2 and Qt 6.1.0.