Drudge enemies (Brazer, Foreman, and Scrapper) stop registering as being inside Grendel's gut almost immediately after being consumed. Grendel gains armor and Nourish or Regurgitate can be cast the instant the enemy icon pops up but it seems like after a Drudge enemy gets damaged by the passive 2% digestion damage just once, they stop registering; and Grendel loses the armor gain and can't use abilities requiring an enemy in his gut despite the Drudge enemies still taking up space.
I tested this in multiple mission types: Exterminate, Sabotage, Capture
With Grendel and Grendel Prime
In and Out of Steel Path
With and Without Companion (Sentinel)
With and Without any Mods (including Aura) or Arcanes Equipped
Swapped Focus at least once
And swapped Weapons at least once
(the last screenshot is from a Grendel with no mods/arcanes/invigoration/companion and only the Ignis Wraith equipped)
So I'm pretty sure it is caused by an interaction with Grendel's kit and the Drudge enemies directly and not some other mechanic/factor influencing the interaction.
