Spawn issues in O.Vallis cave

[(XBOX)Mharcei]

All missions started from Transit Depot (after establishing SU agent) level 10-30 (scorched earth, hunter killer, etc) . Ran into this in 3x different "seek and destroy" phases from 3 different missions from the SU agent. Order of experience:

1) kill 25 drones

2) kill 25 moa

3) kill 25 corpus

Spawn area is limited to inside the cave (fairly small, southwest of transit depot). Leaving the spawn area potentially forces spawn (seemed longer if i stayed in the red radar circle), however the red cursor is located about 109m from red radar circle edge inside a piece of stone wall(?) At the top of a small metal slope on the bottom floor of the cave. Force spawning done with both rapid bullet jumping and archwing movement between red radar circle boundaries. Attempts to kill any invisible spawns with Saryn's 4 unsuccessful. So either the spawn rate is SUPER slow (sometime 1-3 mins between spawns of 2-3 targets) or spawns are arriving in an untouchable location (suspected based on audio). Based on 3 separate phases it is currently 100% not possible to earn the phase bonus simply due to slow spawns; lucky if you're even halfway.

1) 19/25 drones killed. 100% of spawn killed, full 7+min spawn duration 

2) 25/25 moas killed. 100% of spawn killed, 30s of 7+mins remaining

3) 25/25 corpus killed. 100% of spawn killed, 1min of 7+mins remaining

66% chance to complete phase with no opportunity for bonus.

I'm sure this isn't working as intended. I would recommend checking spawn locations, then timers, then confirming bonus is achievable, and then potentially adding to mob size of spawn if needed.