After summoned several entity fallingblocks which have specific blockstates(for example, BlockState:{Name:"minecraft.ice",sand is not creating the same problem) that make the block become item entity after falling onto the players body(this problem will not occur when only the feet is covered by this block formed by the fallingblock, it will occur after the block covers the player's head, and the block at the feet of the player will also become a item entity). After that(ice as the example), if the player fly straightly up for more than 2 blocks, and cancle flying with pressing the space twice, the player will just be able to stand in the air that is about 0.8 to 0.9 blocks higher than the surface under the player, which seems the system believes that there is a block right under the players, but the data command shows players is in a situation of having nbt OnGround:0b. Player will not be able to jump at this situation, and could hardly walk, and if the player destoried the blocks under and goes around to get under the boarder, it will not disappear, and would try to limit the player's flying hight, but it seems to become disappeared after 10 seconds without touching it.
1.create a new world
2.fly to a place that is not really far away from the ground
3.repeat command /summon minecraft:falling_block ~ ~ ~ {BlockState:

,Time:500} and do go away from the blocks under
4.after the two blocks that could cover part of the player's body becomes entity, jump at least 1.2 blocks higher
5.see if that really happened
the time difference between the two pictures will prove that it truely happens in my world, and if it's duplicated then I am sorry that I didn't find it