I just completed The Heliophage three times in a row and for the first two I didn’t even get to loot Rendain because the loot shot out of him in the opposite direction to me, so the 3-5 seconds you’re given before being teleported back wasn’t enough time to reach it.
I recall a similar possibility if you kill ISIC while he’s at a high altitude, where the round can finish before the loot/score even reaches the ground.
If they don’t have an auto collect implemented, then they should definitely look into that. I think it was Destiny that actually have something similar, it picks up any green or higher gear packs that you left behind or missed. It be curious about Rendain’s loot flying off the map too considering the level design
I’m not sure if it’s just a coincident, but after every last boss fight of a mission it says “playername” has picked up the gear, bonus coins or whatever. So it could be auto picked up, or the person I play with manages to pick everything up.
No I noticed this as well even after it says mission completed it sais bonus credits for the team x5 or something. I will try to confirm this tonight when I play again. I do know the yellow small balls disappear.
All the bosses in the Rendain fight have a tendancy to throw all their loot over the edges too.
Doubt they’ll ever put a “vacuum loot” deal in seeing as the bonus scores are in the mix and it seems intended to have to run around and collect as many as possible to up your score. The scoring system at large is incredibly flawed at the moment though so hopefully I’ll be proven wrong and they rethink the whole thing and let you auto collect all the goodies.
I have confirmed that you only need to grab the yellow score the rest will be awarded after the mission is completed. Also the gear you left behind before. So no stress and just get the yellow balls 
don’t know if can watch this but