When you check against removed or undefined attributes it works fine:
✔️ diff({a:{}}, {a:undefined}) returns {a: undefined}
✔️ diff({a:{}}, {}) returns {a: undefined}

But when you invert and add/change to an empty object it differs depending if it was undefined or nonexistent before:
✔️ diff({}, {a:{}}) returns {a:{}}
❌ diff({a:undefined}, {a:{}}) returns {} instead of {a:{}}