i just tested it out with the default settings of platformer, it acts the same if i move.
it's probably ur acceleration being higher than the deceleration make sure they are equal.(that is why i asked for a screenshot or setting values from your project, it might be some settings are not properly set)
however u can override the default settings by gapping sort of speak the max jump to a surtain value by changing the player.Platform.VectorY
and say in ur conditions on jump or
up arrow/w key pressed player set Self.Platform.VectorY-(your value)
default jump strength is
650 (that is not taken in account the moment u use the above custom vector.y) if you want to lower it just change
-(your value) to desired strength.
u might also want to disable the default controls and create custom ones by events like i explained above.
if it's not fixing the issue, it's probably some event in ur project that interferes with acceleration / deceleration /gravity / game time scale.
if u want to revert the settings of platformer behavior without removing the behavior and you can't remember what it was by default here are the values