I will need to look into this. I don't remember if the movement code is so old it uses Game Maker's built-in collision system, if that's the case it might explain what's happening (colliding with a solid object cancels the ongoing movement and requires you to manually reapply it if you wanted to get as close as possible - like sticking to the ground when landing).
Stumbled across yet another graphical inconsistency regarding a player-character's footwear: Yal's disappears while using the Cat Sunflower's climbing ability:
This one's extra surprising because I'd imagine most people would play as Yal since he's first. Maybe they assumed Yal needed to take off his sandals to climb, but seeing as how this doesn't happen for Matt, Anikel, or Paige, I think it's safe to say this one wasn't intentional, either.
By the way, remember how I said that using the Hikkori Skull lets you move around while talking to NPCs? Same happens if you use the Rocket Pack.