If ur shapeshifted inside of a building and try to shift to travel form after leaving, u will still have “you cannot use this inside” error until u stop moving outside. Preventing seamless shapeshifting, especially to flight form.
Welcome to 2007.
WoW doesn’t update your position server-side immediately - there’s a small time lapse. It’s not a long lapse, but it’s enough that you as a player can easily be quicker than it (especially with some latency involved). As a result you can push the shapeshift button at a time when your client shows you outside but so far as the server is concerned you’re still inside.
You can force a more rapid positional update by either stopping moving or by momentarily strafing. Either will let you immediately shift. You can also delay it further by jumping, though I’m not sure why you’d want to do that.
Note that this also comes into effect when dodging or entering area effect mechanics.