I think I’ve seen another aspect to this bug - but it might be unrelated.
When Shared Edits is turned on for a post, if it is edited too soon (within 20 seconds or so), then a clash seems to occur where only one or the other edits is saved. In other words, the Shared Edit functionality doesn’t actually kick in despite it appearing to be active. Things get very confusing if either author close and re-enter the post again, with edits appearing and disappearing.
If left alone for a while everything seems to sort itself out (albeit with loss of some content). Perhaps a brief 30 second lock of the posts when shared edits are turned on would prevent this?