For the OPQ Cellion, it respawns once per platform in a pattern after it's killed (plat order: 1, 10, 9, 13, 11, 6, 12, 2, 5, 15, 8, 4, 7, 3, 14). Only the last one (on plat 14) drops the piece. The respawning breaks for any one of them, not just the last one that drops an item. So far, I haven't encountered the last one not spawning and/or not dropping the piece. For me, the one on the 12th platform didn't spawn during 2 PQs, i.e. party killed it on the 6th platform, then it never respawned on the 12th platform (or any others). It also happened once when it was supposed to spawn on the 2nd platform. Just wanted to clarify it seems to happen for any of them, not just the last one that drops a piece.
Also, it seems to happen more often than I originally thought. It happened 3 times today out of about 10-12 PQs, and happened to other party members a few times before that. If nothing can be done atm, I guess doing that stage early on to be safe is the best alternative.