fix: never reuse a collection entry index after a removal

assets/controllers/form_collection_controller.js

@@ -11,10 +11,13 @@ export default class extends Controller {
 
     connect() {
         this.itemsCountValue = this.fieldTargets.length
+        // A running counter rather than the current count: after a remove, count + 1 can be an
+        // index a remaining entry still holds, and the two would submit into the same slot.
+        this.nextIndex = this.fieldTargets.length
     }
 
     addItem() {
-        const index = this.fieldTargets.length + 1
+        const index = this.nextIndex++
         const prototype = JSON.parse(this.prototypeValue)
         const newField = prototype.replace(/__name__/g, index)
         this.fieldsTarget.insertAdjacentHTML('beforeend', newField)