I have never had that happen, but Acrobat is just glitchy enough that I've learned to save my work early and often to a new file name so that I can backtrack if needed. Not much help, I know, but saving to a different filename should preserve your previous version.
I have had this happen. It's odd. Some things seem to get saved, while others do not. I haven't tracked it to be certain, but as I recall Bookmark changes or changes to File Properties, for example, won't be saved while working in the Content or Tags panel may be. No idea why this is the case.
As mentioned above, it's always a good idea to make interim Save As's using a different file name (File01, File02, etc.) so if your document should become corrupt during the editing process you have an earlier version to go back to.
Thank you. That is what I have been doing. It only does it some times. Really annoying.