How are you creating the "lip" of the pipe? As part of the background? As a door bubble?
It's hard to figure out what's going on here, but I'll explain how the doors work and maybe that will help figure out what is going on. Suppose you place a door bubble in a room. When the room is loaded, a vertical strip of six tiles (where the door bubble will go) is erased and replaced with tile $4E. It looks like it is done this way so that background collision detection can be used to make the doors solid/shootable. 4E tiles also appear to behave as solid, except when placed where a door bubble goes.
So, possibly, if you made your door bubbles invisible, and made the lip of the pipe using background tiles, and tile 4E was solid white, that would explain what you are seeing. But that wouldn't explain why you don't need to shoot the door bubble.