From what I understand, loose lists are a basic feature of markdown and cannot be disabled in Obsidian. I like to put line breaks between lists sometimes, and I don’t want my regular lists turning into one big loose list when I do that, so I searched for workarounds and found this to be the most elegant.
You can simply insert a braille space (⠀) in the line break between your lists, and you will get two lists separated by a normal-looking line break. I found other workarounds, but they either insert visible characters into the line break or cause Obsidian to treat the separated lists as one list (which sort-of breaks bullet point relationship lines). Other exotic whitespace characters (like zero-width spaces) don’t seem to work either, not in desktop live-preview mode anyway.