I just spent the last 12 hours or so grinding through the campaign - somewhere along the way during Act 1 I was not given credit for my own personal accomplishment of the quest line. Thinking nothing of it I decided to continue along anticipating finishing it later. Fast forward, we complete Act 3 and now we are supposed to have access to our mounts. My friend has access no problem to his mount, however for myself the quest shows as completed but I cannot access the mount.
So I go back into my own lobby and run through the rest of the “missed” A1 until I get it right to the end of A1. Upon completion basically what I expected happened it shows that my next main story quest is in A4 which would be great EXCEPT when going to try and use a mount, it still tells me I need to complete the specific quest (aka the last one in Act 3). A quest I’ve already completed and now have no plausible way to re-do without running through basically the whole damn game again with someone else.
Ultimately, this is a bug - there is something wrong with how the game is identifying the completion of quests on individual milestones when connecting it at least to this specific unlock requirement and I think it needs to be looked into further by developers. In the meantime I just hope I can find someone who is close to that point in the main story to get past this hurdle and hopefully get my mount.