I only changed one quantity, but the document status changed from 'In Discussion' back to 'Editing'. Is this normal?

Yes, this is normal. In Patisco, both parties can modify their own copy at any time. Once you make a change, it returns to 'Editing', which means "this version hasn't been sent to the other party yet". Documents won't be locked just because both sides have reviewed them. If either party makes further modifications and sends them out, a new round of negotiation begins. Remember to click 'Send' after editing so the other party receives the new version and their copy updates accordingly. Exception: Archived documents cannot enter edit mode; all write access is disabled. However, you can still view them, print them, and use the discussion feature. For orders, 'Unarchive' will return it to 'Editing'.