The regression is that for (oddly only) one of the models, which has a <range> animation in the .xml file and an aircraft cache file (in the aircraft data folder) from a previous FG version is present, these particular parts don't show up.
Once the range animation is removed and the old aircraft cache file as well everything shows up.
Needless to say this only happens with 'next' / 2020.4.0 (hence regression).
Eric