Bug Alert,
This works with simple object for example draw a rectangle and make it a symbol, the same behaviour will persist.
With a hybrid symbol, open "multiple views panes".
Edit the 2D component of the symbol, the 2D objects disappear.
Also happens when editing a symbol containing symbols, if a symbol is 2D only it will disappear when entering the root symbol with multiple views on.
See Screen recording.
Very frustrating behaviour!!
Screen Symbol.mov