When you make player A spectate entity B using the /spectate command, but they are in different dimensions, then player A will teleport to the correct coordinates, but not the correct dimension and won't actually spectate entity B.

How to reproduce

1. Spawn an entity (or get another player), and then go to different dimension at different coordinates
2. Switch to spectator mode
3. Use the /spectate command to spectate the other entity

Now you and the entity will be at the same coordinates, but in different dimensions.