Following @sorvell suggestion

Author: LostInBrittany

src/lib/updating-element.ts

@@ -506,18 +506,15 @@ export abstract class UpdatingElement extends HTMLElement {
       const needsFirstUpdate = !(this._updateState & STATE_HAS_UPDATED);
       this._markUpdated();
       if (needsFirstUpdate) {
+        this._updateState = this._updateState | STATE_HAS_UPDATED;
         this.firstUpdated(changedProperties);
       }
       this.updated(changedProperties);
     } else {
-      this._markNotUpdated();
+      this._markUpdated();
     }
   }
   private _markUpdated() {
-    this._changedProperties = new Map();
-    this._updateState = this._updateState & ~STATE_UPDATE_REQUESTED | STATE_HAS_UPDATED;
-  }
-  private _markNotUpdated() {
     this._changedProperties = new Map();
     this._updateState = this._updateState & ~STATE_UPDATE_REQUESTED;
   }