Use a safer flag, based on internal testing.

lib/mixins/property-effects.js

@@ -2801,8 +2801,16 @@ export const PropertyEffects = dedupingMixin(superClass => {
       dom.templateInfo = templateInfo;
       // Setup compound storage, 2-way listeners, and dataHost for bindings
       setupBindings(this, templateInfo);
-      // Flush properties into template nodes if already booted
-      if (this.__dataReady) {
+      // Flush properties into template nodes; the check on `__dataClientsReady`
+      // ensures we don't needlessly run effects for an element's initial
+      // prototypical template stamping since they will happen as a part of the
+      // first call to `_propertiesChanged`. This flag is set to true
+      // after running the initial propagate effects, and immediately before
+      // flushing clients. Since downstream clients could cause stamping on
+      // this host (e.g. a fastDomIf `dom-if` being forced to render
+      // synchronously), this flag ensures effects for runtime-stamped templates
+      // are run at this point during the initial element boot-up.
+      if (this.__dataClientsReady) {
         this._runEffectsForTemplate(templateInfo, this.__data, null, false);
       }
       return dom;