OBD - Undo button undid EVERYTHING since I opened the file

On Friday afternoon I spent about 30 mins doing drawing edits.

I had to undo a single line placement and it instead undid everything since I opened the file.

I tried clicking redo and it all came back....

I thought this really odd, so I saved the file and closed it.

I re-opened it to test and 'undo' appeared to work as normal, ie the undo button would only undo one step at a time.

I continued working for another 30mins, went to undo something, and again, it undid everything for the last 30mins, only this time it crashed in the process. When I re-opened the file I'd lost 30mins of work.

I understand there is now an option to "set a mark" in the undo tool - however I have not used this.

I have also been using the undo tool from the "standard" toolbox menu, not the ribbon menu.