blockGap, and the margin you stopped writing

Vertical spacing between blocks moves from a stylesheet rule to a setting, which is a better mechanism and a large diff.

{
  "settings": { "spacing": { "blockGap": true } },
  "styles": {
    "spacing": { "blockGap": "1.5rem" },
    "blocks": {
      "core/group": { "spacing": { "blockGap": "2rem" } }
    }
  }
}

/* generated: --wp--style--block-gap, applied through
   :where() so a theme rule still wins */

Setting blockGap to false disables the control and stops the CSS being emitted, which is what a theme with its own spacing system wants — the two together produce doubled margins that nobody can find. The generated rule uses :where() and therefore has zero specificity, which is a deliberate choice that makes overriding it easy and makes it invisible in devtools.