I didn't even know that this was a problem or a step in finishing the cipher as I just got my first one today. I can certainly tell you though that I'd be pissed after reaching that point only to be forced to sit and wait for no good reason. One of my first exotic bounties ever required me to purchase something from Xur but I didn't know it at the time. Got the bounty Sunday evening. Slammed it out on Monday after work. (Or at least I would have if not for Xur making me wait a week!)