The bug

The teleport command fails when teleporting the player above the limit of 20000000, but it does not fail when teleporting a player to a destination with a lower height from which the player was teleported, despite having a destination at a higher height than 20000000.

How to reproduce

1. Run /tp @s ~ 19999999 ~
2. Go up until the player is at a higher height than 20000001
3. Run /tp @s ~ ~1 ~ (the player will get the message "Invalid position for teleport")
4. Run /tp @s ~ ~-1 ~ (and this time the command will work, despite the destination still having a higher height than 20000000)