The bug

Sometimes, the game engine will "forget" about some teleports for a while, then will do them all very fast to catch up. This occurs more often the faster the teleport command is being run. It also occurs more often when the player is far away from the command blocks (probably caused by MC-74963). I haven't done much multiplayer testing, but it seems that other players can only see the glitching on LAN, not regular servers.

How to reproduce

Run on a 20 hz clock:

execute as @p at @s run tp @s ~ ~1 ~

→ The player will not move steadily upwards. They will move up at first, then fall down, then start teleporting up again. This will keep happening indefinitely.