I can’t notice a pattern, but it happens all the time in both instances I use. I get to a post and there’s an image that can’t be loaded. I open the post on the browser and it loads fine.
(I’d share a screenshot of the browser but the inline picture upload feature is giving me grief now. ☹️)
Here’s the sample post: https://feddit.org/post/538582
It doesn’t seem to be contained to that specific site.