WowAce

Use the same profile for all characters.

Compress a category by clicking on it so that appears at the bottom.

Relog to a character which doesn't have anything which matches the category which is compressed.

Result depends on how many sections are not matched and what order they are in, but always there is a large empty section at the bottom, sometimes with (matched) compressed categories floating in the middle of it.

I'm uploading a screenshot of a simple example.

Image 1) Shows my main char with 3 compressed categories.
Image 2) Shows my alt character whcih also has things in those 3 compressed categories. Works perfectly
Image 3) Shows me compressing the Category 'Cloth' on my main character.
Image 4) Shows what happens when a compressed category has no matching items on my alt. Weird blank section at the bottom.