Same Issue. Multiple opponents.
After the 4.19 patch there is a issue with Azir's ult not blocking or knocking back champions. Last game I played, the ult failed to push back or block opposing champions that were moving. It happened against a Kat, a Yasou, and a Voli. They were moving toward me (Azir) on Summoners Rift when the ult was used and the opponents kept on walking and went right through the wall. The ult was far enough behind me and ahead of them to catch and push them away. All three champion glitches were at different times of the match. The wall did work correctly on non moving champs 1 or 2 times.