The mythic quest is currently broken and the upgrade quest cannot be activated unless you have all the completed quests from the process in your log. The downside to this is that the quests are all reset if you ever handed in a new base weapon following the completion of your first mythic. Or, in rare cases, people that were given mythics without doing the quests.

They likely wanted to avoid the same situation as before, where mythic users could not complete the quest line if they had started to upgrade their weapons, but they botched it. I bet this will be patched, but probably not immediately.