404 Page Being Shown While NextJS Page Is Rendering - next.js

Question

I build all of my initial routes for a dynamic slug based on what I have available in my CMS at build time. Then as new CMS data is available, I will generate new pages when they are requested by the user.
I'm running into an issue where a user might hit /posts/1, but since posts/1 has no CMS data, I'm rendering the 404 page using the notFound boolean inside of getStaticProps. This is what I expect to happen.
Now, if I publish some data to /posts/1 in my CMS, the next time a user hits /posts/1, the API data is fetched in the getStaticProps method, and the page begins to build. The problem is that the user is immediately redirected to the 404 page even though the notFound block is not hit. The router.isFallback flag is also shown as false when it should be true since the page is building. Now, if this user were to refresh their page, they would see the generated page and no longer the 404 page since it is now built.
It seems like Next might route the user's request to the 404 page in the second scenario since it was previously a 404, but is there some way I can "block" the user's request or show my fallback until the page is rendered so that they don't see this 404 when the page is building? This scenario would not happen if the first user did not hit this slug before there was CMS data available.
Here's a simplified example of how I am using getStaticPaths and getStaticProps:
export const getStaticPaths = async (ctx) => {
const myInitialPaths = await fetch('http://my-cms-endpoint.com/initial-paths/');
return {
paths: [
...myInitialPaths
],
fallback: true
}
}
export const getStaticProps = async ({ params }) => {
const myPageData = await fetch('http://my-cms-endpoint.com/page/#id');
if (myPageData) {
return {
props: {
myPageData
}
}
} else {
return {
notFound: true
}
}
}