Looks like a sweet MegaMan X clone you have going there :). The gravity and jump in the video, reminds me of underwater areas of the original game.
When bugs crop up, I usually try to think of what I most recently added into my game, that may conflict with game play mechanics that were working previously.
I like keeping backups of each project, saved at different stages of development as well. So I can go back and review any changes I've made over time, or see how I was performing certain actions the day or two beforehand. If something strange happens along the way.
It's handy to use a collision box for the platform behaviors, then an animated sprite moved overtop and set as a child of the collision box at the start of layout. Using the collision box for any behaviors & collisions, and the player sprite on top for all the animations during various conditions. Especially, if the frames of the Player Sprite, Enemy Sprite, etc, change dimensions over time.
You can somewhat see what I mean in my YouTube video here: https://youtu.be/kHscxNDZqXU . There is some debug text used to keep an eye on the Player's State as well. As you can see the enemies do not use a collision box below the animated sprites. This is because I am sure to keep each frame of every animation the same dimension. So when I set a bounding box, origin point, and image points to every animation. They will be at the exact same location in each animation and frame of the enemy object.
Hopefully that makes some sense. Sometimes I feel like I'm just rambling. But, I'll have a tutorial series soon, where folks can follow along and recreate what I've got going in Teddy's Adventures.
Like lennaert said, there's probably a simple conflict somewhere in your event sheet(s). This happens to me all the time. There may need to be an extra check somewhere to see that you are not jumping/falling, but are on the ground. Then if so, set back to idle/walking states depending on if the keys used to move around are being pressed or not.