Oh, I'm glad I searched for this topic before making a new one. I have more information on this particular bug for such a great Warframe:
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This bug only occurs during Pulverize. If you exit Pulverize while the bug is active, then re-enter Pulverize, the bug goes away.
The bug's effect is that, after landing from a medium height or greater, all of Grendel's horizontal movement during Pulverize will completely halt, leaving him stuck in place and unable to really move. Vertical movement seems unaffected, since he does a tiny bounce during the bugged halting effect. The height needed for the bug is less than is necessary for a hard-landing animation - perhaps it's tied to the earthquake mechanic? Perhaps not? I vaguely recall not triggering an earthquake, but still getting the bug.
The bug can be reproduced by entering Pulverize, then casting either Feast or Nourish. (Not sure if Regurgitate contributes to the bug, because I always subsume over it.)
The bug will even stack if you cast BOTH Feast and Nourish during the same Pulverize session, causing Grendel's horizontal movement to halt for a longer period of time!
Grendel's current gut contents is completely unrelated to the bug, as far as I can tell. If I had to wager a guess, it's the fact that Feast and Nourish normally have a cast animation when not in Pulverize, where both animations prevent Grendel from moving horizontally.
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This is a particularly annoying issue since it forces me to constantly spam Pulverize whenever I want to play Grendel really fast. If I've ever triggered the bug - and I know when I have, since the bug reproduction I've listed works 100% of the time - I always make CERTAIN to exit Pulverize before I land on the ground, then enter it again right away! So I'm constantly working around this bug whenever I play Grendel.
Also, I feel the need to mention that I enjoy playing Grendel with both the Tractor Beam mod for pet MOAs, plus Motus Signal for a stronger double jump. This extremely high jumping height thanks to the lower gravity could be a factor? Though I haven't felt the need to do further testing myself.
Unrelated quirk: Grendel can walljump during Pulverize if you meet the conditions. It even resets his double jump! Please keep this quirk, DE. I love its goofiness.
Here's hoping DE sees this post, and fixes Grendel just in time for the upcoming major update.