Some Exterminate missions still dont have enough enemies spawned to complete

[darknite323]

Corpus outpost tileset in this instance.

Sortie 1, doing Exterminate on Venus, Unda.

Ran out of enemies, 15 short of the target:

This was at the 3 minute mark.

We had to wait a whole 4 MORE MINUTES, to get all the spawns needed to complete the mission because they decided to only spawn in 2 at a time until the end when finally 7 or 8 spawned in at once.
So to clarify, what would have been a 3 minute mission, took over 7 minutes because of bad spawns.

This is not the first time I have seen this happen, seems mostly to occur on this tileset from what I can recall.