Ok, one more update from me. I just updated MB Relationships on our production server too. Tested by updating one of our speakers, and then checking the event, and that speaker was bumped to the bottom of the list.
However, after dragging the speaker back to the correct position on the event page, updating, then updating the same speaker again, the order was preserved. And it’s preserved every time now for that event.
I tested our other events, here’s what I found (that mirrors what Clayton notes above)…
Any new relationships that get created with the updated plugin code should preserve order, because of the way index order is getting set now. But for any existing posts that had relationships defined before updating the plugin, you must go into each of those posts, and update each post once to establish an index order for its related posts. Then specified post order will be preserved from that point forward.