Update #5: another two months have passed. So I'm concluding as Xirev says below that this is indeed just RNG and once you've maxed your mission table (still level 20 in patch 9.1 I think) you should do the Maw missions whenever they're up. My change of covenants meant my new followers were too low level for a few weeks to really farm this achiev, but I have gained one more of the missions this week (I have two L60 followers and the rest L59 down to L8). Also, I've raised a ticket but GM just says no known bug Maybe that will help spawn the needed adventures. Interestingly although it's offered me a Maw adventure from the top 4 again, it's also offered me advancement (my Venture Plan addon says I'm now on 21). I've changed covenants from Night Fae to Kyrian. My new working theory is that a very specific follower needs to reach L60 to move me on Update #2: week 7 since levelling my first follower to L60 and still only getting missions from the first four. FYI: other than the 9 at L60, today my followers

Adventures into the breach download