OK, here are the steps leading to the bug I have found.....
-With assembly loaded lightweight, click on a part within the assembly and click open part.
-Make edits to part (or just look at part for reference) and close part (with or without saving).
-When assembly comes back up, the part that was opened and closed has vanished however, it is now resolved.
Notes: This happens to lightweight parts only. Resolved parts don't disappear after performing the aforementioned steps.
Toggling suppression brings the part back.
File -> Reload will bring the part back sometimes, but not only.