They don't lose the mouth because they're heavily inbred, at least not as a whole. They lose the mouth because those dogs haven't been selected for that trait for some generations.

If you take dog A that has a heavy mouth, who is NOT from a line of dogs noted for that, then you're not going to recreate that over and over inside of a few breedings. You have to SELECT that type of mouth over and over to solidify it along the way. Without doing that, it won't ever be where it needs to be.

If you take dog A that has a heavy mouth, who IS from a line of dogs noted for that and has produced it consistently, breeding dog A to other females that have the same trait will get you dogs like you want.

If you heavily inbreed on dogs without selecting for that trait, it's going to be lost. That is the key. Selecting dogs with that trait. It's not the inbreeding; it's the wrong selection for the most part.