I Think i found out why the bullet bill glitch softlocks on all-stars. What happens in bbg is Mario clips into the base of the flagpole but clips low enough to get pushed but he still triggers the end of the level. The Game requires you to after the level starts counting down, to touch a block and it can be any block. That is why when you jump over the flagpole there is an invisible block next to the door. My point is that many things were fixed in the all stars version such as better Hitboxes and various despawns. My theory is that they changed it so the block that needs to be touched is the wall tile and not the stair texture. One time i triggered it by accident lol in a warpless run

I think it softlocks because for the all-stars version its not just mario simply walking to a wall to end the level, but the castle has to be entered so that it triggers mario to do an animation that ends the level.

My theory is it's probably checking for an x-position instead of just simply hitting a block. The block just puts him behind the layer (which is why bbg technically does work) but the game only loads after he reaches a certain x-position.

BBG only works on the Japanese version of all-stars. Because the game thinks that Mario is getting pushed into the base of the flag and it still works and you can continue. This doesn't work on the American version though.