explanatory comment

packages/svelte/src/internal/client/reactivity/props.js

@@ -322,7 +322,15 @@ export function prop(props, key, flags, fallback) {
 	} else {
 		getter = () => {
 			var value = /** @type {V} */ (props[key]);
-			if (value !== undefined) fallback_value = /** @type {V} */ (undefined);
+
+			if (value !== undefined) {
+				// in legacy mode, we don't revert to the fallback value
+				// if the prop goes from defined to undefined. The easiest
+				// way to model this is to make the fallback undefined
+				// as soon as the prop has a value
+				fallback_value = /** @type {V} */ (undefined);
+			}
+
 			return value === undefined ? fallback_value : value;
 		};
 	}