I've seen this (or a very similar) issue multiple times on the desktop website when I have a post open for an extended period of time without activity, for example if I leave a tab in the background or leave my computer for a while. I'll come back and it will be the same URL but different post content. If I refresh the page it shows the correct post and comments.
I haven't noticed this when I'm active on a post, whether I'm commenting or just viewing (scrolling, voting, etc.). I use Firefox on desktop but I have noticed this on Chrome mobile browser. I also haven't had an issue with initial post opening or commenting.