@james - I think this is a bug, or at least sub-optimal behaviour!
MC6MkII. Was on 3.8.6, updated to 3.9.0. When I look at one of my bank preset messages (CC #103, value 0) that existed before the upgrade, the new ‘use value from expression’ toggle was set on. Easily switched off, but guess most users wouldn’t expect it to be set on?