It sounds like the platform is set up to drop a bit when you jump on it, now the problem is returning to the “hovered” height before jumping on.
You will need something fast fire like an Event Tick or Set Timer on Loop .01 constantly checking the current height of that platform. Is the player on it or off it? And if so do this or this.
Try setting a boolean when player is On the platform. With fast fire go to a branch with that bool and if not true set it to your desired hover height.