After playing Kahl's new weekly mission "Prison Break" five times, I've only been able to find and free precisely 4 out of the required 5 veiled Grineer every time, despite always searching the map exhaustively.
It's maybe also worth noting that they spawn in different locations, sometimes all four spawn before the dropship ride, sometimes one spawns afterwards, the only real through-line is that it's always four. The map is also different every time, with other objectives reliably spawning in random locations and in adequate numbers.
Unfortunately, I can't say for sure what causes this bug, and it doesn't seem to affect many players. However, I was able to find some other players who seemingly had the same issue and insisted they had searched the entire map.
