onground only depends on y translation, and therefore can be set to true even when x,z translation moves the entity off of the ground.

Solution: recheck onground after the move is complete so that the next move does not allow ground-based interaction (jumping, etc.) when the entity really isn't on the ground.

This is a client-side issue. The client calculates onground immediately after the Y portion of a move, which occurs before X and Z. This means that a move can (and often does) move a player off of a block and into midair, while the client still thinks it's onground. Since onground is only calculated during a move, this means that the next move will behave like it's on the ground and will allow jumping even though the client is in midair.

This is nearly impossible to see visually, but it translates to odd movement mechanics server-side making anticheat systems use convoluted workarounds ("player COULD have hit the ground during the Y portion of this move so they might not be cheating").