You can maybe try something like this,
in this specific event order:
>>> set player Y position to Y("player") +1
player overlapping backdrop
player overlapping active ground
>>> player is on ground
always >>> set player Y position to Y("player")-1
in short, we bring the player 1 pixel down,
if we detect he's overlapping backdrop or active (ground)
it means he is "over the ground"
then we place him back at its original Y position
here's a quick example:
this approach would not work as precisely if the "active ground" is a moving object,
due to the overlap timing issue in Fusion (moving ground position will be tested as it was from previous frame)
if your "active" ground is fixed you won't have any issue though!