Sometimes on 1-3 when I go behind the end of the level it doesn’t teleport me into the room with the whistle. Does anybody know why it’s doing this?

You might be jumping over where it takes you to the area. Once you go behind the stage don’t jump at all

^^^ this

Just don’t jump when you’re in the black area and you should be fine.

You might be opting to continue after dying, at which point the whistle wouldn't be there.