itch.io

That's interesting. It looks like you're not in the "full run jump" state at all when you jump right: both the speed and the animation suggest a normal jump. Could it be an issue with 3-key rollover or something? The directional or jump button being "released" before the jump is triggered (due to rollover) could potential cause something like that, but I'm not sure about the asymmetry between left and right, and it contradicts the keyboard visualizer (which is very helpful, by the way, thanks for going the extra mile to put it there).
I'm just speculating here, since there isn't anything in the code for jumping that treats left and right differently. It's not easy for me to understand what's going on, so I don't know what to say. If possible, maybe try a different keyboard or computer, and see if the issue persists? I'm sorry that I can't think of anything more helpful right now.

EDIT: looking more closely at the visualizer, it does actually look like it's letting go of the 'x' button as you press 'z', in both directions, but maybe the 'x' input lingers a bit more when you jump left, thus not exiting the run state until you're in the "jump" state? So it could really be a rollover issue, possibly?