The TP command (Teleport) seems to be working suspiciously.

When I tp the player 2 blocks above EVERY TICK, it just makes the player bounce. It seems to tp him, then making him fall, and once he hits the ground, it "launches" it in the air again, I remind you I tp the player EVERY TICK.

It seems that the "teleport" command only works when the player doesn't have a Y axis motion.

How to reproduce

Take a repeating command block, and put inside of it : 

/execute as @a at @s run tp @s ~ ~2 ~
then activate it. You should start to go up, then fall, then go up... and not teleport up.