I had no such issue, nor have i heard of one. With that said, there are a few things you can check/try. Firstly, make 100% sure you have cleared level 50 in the original PotD. If possible, make sure any associated quests are turned in as well. If these are done you should have the option to que in starting at floor 51, which you can immediately cancel(thus making this a very quick thing to check). A quick character search shows you have the reward for finishing the MSQ step required, so thats set. The next thing to check is that you are looking for the quest on a class that is at least level 81 - sometimes quests dont show up unless you can currently accept them. And finally, id make sure i hadnt already done it and forgotten. This isnt meant as an insult in any way, but a common reason on the forums when someone says they cant do something is that theyve already done it and forgotten(or picked up the quest and not advanced it). If none of those help then i am currently out of ideas, but maybe someone else will have a suggestion ive neglected.