docs(faq): add onMounted ref question

closes #19736

packages/docs/src/pages/en/getting-started/frequently-asked-questions.md

@@ -101,4 +101,8 @@ The following responses are a collection of common questions asked by the Vuetif
 
   Not directly, you have to set `configureCompat({ MODE: 3 })` globally and `MODE: 2` in each of your components that you want to run in compatibility mode. There will still be some incorrect warnings that can be ignored with the `-ATTR_FALSE_VALUE` filter in devtools.
 
+* **Why don't elements exist in the DOM yet in `onMounted()`?** ([#19736](https://github.com/vuetifyjs/vuetify/issues/19736))
+
+  Some vuetify components are asynchronous, so there is no guarantee that their children will be mounted immediately. Either wait for the target element itself to be mounted with `v-on:vue:mounted` or use `onMounted` in a separate component that only has plain elements between the `<template>` and your target element.
+
 <PromotedPromoted type="theme" />