changed labels are reverted if another user saves an already open draft

3 cases happend within 5 minutes:

• user 1 opens a page with no labels and begins editing
• user 2 adds a label to the page
• user 1 saves page
• user 2 refreshes page: there are no labels

• user 1 opens a page with a single label begins editing
• user 2 removes that label and adds 2 other labels to the same page
• user 1 saves page
• user 2 refreshes page: there is now just the original labels

• user 1 opens a page with a single label (with 3 labels)
• user 2 removes the single original label but leaves the 2 other labels to the same page
• user 1 saves page
• user 2 refreshes page: there are now three labels again