Welcome new person! o/
That's an interesting problem. It's weird it works on one but not the other? I tend to play with controller, but the "cancel" button also is "go down" (when flying or swimming). Another thing is if you point the camera down and the walk forward in that direction (I guess on mouse and keyboard this is hold both mouse buttons down?), it will have you dive as well.
Might also check keybinds like the person above since you might somehow have gotten one character's specific keybinds changed somehow?
I'm not a super techy person, but those are just some thoughts for things that might work? Hope you can get it working for ya!
