Note: fixing this issue will cause quite a mess for all the cases where the merge is conveniently hiding concurrent changes done by the same user in the same editing session, but not updated properly in the previousVersion metadata (for example, when uploading a file to the page as a result of a manipulation of the wysiwyg or of the attachment selector macro, if there are still such cases).

Basically, this bug reproduces only for the cases where the edit form contains that were impacted by the merge.
For a structured page, for example, edited through a sheet, the form of user A could only display some fields from the page and all other page metadata could be safely updated in a concurrent form & merged on save, not sure how to detect this.