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One thing I did notice is that those first already published posts did not update the related posts section as more posts were added. I even tried updating it a few times, removing the blocks and readding them.
What I would expect is that the related posts are always changing as the site grows and more content is written. I expect this would happen without having to ever go back to the older posts to "update" the related posts content.
As per @MichaelArestad's comment: