I thought this might be the case, as the first time it happened I was making my way across the map to the bank and I ran across one of the boss spawns, defeated it and the message appeared but no location.
However last night I went straight to the bank (after wiping some fixtures and fittings for extra time), completed the robbery and the mission. Then went back across the map, clearing PPD spawns and the boss/minion spawns as I went. Still encountered the same issue, message and no location popping up for the side mission.
One thing I did notice was that the PPD spawns and boss/minions did in some locations have a tendency to interact, possibly with some of the minions getting wiped by the PPD before the bank robbery was done. This might cause the boss spawn to become invalid?
Cheers for the reply though, just wanted to check in case it was the case that the PPD break out was not marked intentionally, etc