Got exactly the same problem right now. Never even seen this POI before, started a tier5 "Infested Clear" from joel, made it in and got super overwhelmed so I ran around the building trying to take them out one by one. Got through the whole thing after I don't know 700 9mm and 600 .44 in 12-14 ingame hours to find out the quest wouldn't finish and no yellow dot would appear. So I searched the whole building a couple of times and destroyed many walls to find the last zombie but with no luck.
I really hope this gets fixed, was not the first time this happened to me in a21 (with other jobs, it was some gasstation POI with a laundry, a kitchen and a restaurant i think).
Best regards