It actually does reset your velocity on respawn, but what I hadn't thought of until now is that I should have ignored player inputs for a few frames after respawn. If you don't let go of the movement keys almost immediately after respawn then you will run off. Thought I had this problem taken care but there's always something else! And you're right, the character's slippery floaty movement was very intentional but including some precise jumps with this kind of movement was not the best decision. It was originally a combat only game but I felt it might be too repetitive and added some platforming sections to try and make it more varied.
Fever mode is what made you a killing machine. Infection is gained while in fever mode and activated after the mode ends. Infection reduces your movement speed, jump height, and attack speed. However, I made infection go away much quicker than initially planned as I was scared of annoying the player if it lasted longer. I guess I overdid it, made it so quick that you didn't even really notice the downsides.
Thank you so much for the very valuable feedback. All useful things to keep in mind with my next game. I'll be sure to check your game out soon!