I've just been trying to save some elevations from a 3D drawing and I had a disaster which I now remember has happened before in an older version (running on 2020 now)
The first thing to remember (and I didn't) is that when you save a 2D projection the file thinks it's been saved - so your undo history is gone, and unless you save it again before you close it your 3D file will be gone too - you'll just have the 2D projected file instead!
So, I set up for the first elevation which involved vector converting a boundary wall so I can 'see through' it but still see the outline when I save the 2D file in hidden line mode.
I saved the 2D projection which seemed to go well. (with hidden line removal and hidden line cleanup active)
I then rotated the model 90 degrees (still looked good) and attempted to save another 2D projection.
Suddenly nearly all the model had disappeared. Most layers were gone, just a few scattered walls and windows remained.
No undo was available and the original 3D model was gone (see the first note above).
Not only that - the first 2D projection which seemed ok, and displayed ok in the thumbnail when I went to open it was also missing all the same bits. (they just hadn't disappeared from what displayed on screen at that stage).
Has anyone else had issues saving 2D projections?
A little more investigation...
This was the first 3D I'd constructed with solid surfaces enabled (see my other post) and it seems that the entities that disappeared were all the solid surfaces. The only solids on the drawing that remained were solids I'd needed to edit by dragging points, and those were converted to solids.
Another good reason NOT to use solid surfaces it seems!