Am still getting this behavior on latest beta, following opening a comment thread on Hacker News.

Going back in navigation history, than forward again into the same thread, via mouse side buttons, produces the correct behavior.

But if I re-enter the thread by clicking the link that navigates to it, I get the broken favicon.
Here's a capture demonstrating:
- navigating to the thread via click
- navigating back to front page via mouse side button
- navigating back to the thread via mouse side button