Yes, it affects 18w03b.
When spectators is inside a player, and the player goes into a portal,weird bugs happen.
/summon wolf ~ ~ ~ {Passengers:[{id:wolf}]}
This doesn't affect the ender dragon.
It happens on my computer. When you try to break a block, the block will lag, and game keeps outputing "Can't keep up! Did the system time change, or is the server overloaded?Running <number>ms behind, skipping <number> tick(s)"
Still in 1.12.1
I relaunched the game and everything was fine.
I also have this bug before. version 1.8.9
Looks like MC-315