Yeah, this is a known pain point — expanding large diffs can mess with the keyboard focus, basically because the DOM gets updated dynamically and the browser loses track of where focus should be.

A few things that might help:

• Click somewhere on the diff after expanding it to "reset" the focus manually before trying to navigate with the keyboard again
• Try pressing ? to open GitHub's keyboard shortcuts — sometimes just opening and closing it helps restore navigation
• The n and p keys (next/previous file in a diff) sometimes work better than arrow keys when focus gets lost
• If you're on Chrome, try the same PR on Firefox — keyboard focus handling differs between browsers and it might work better there

Also worth trying: collapsing and re-expanding the problematic file, which can sometimes reset the focus to a predictable spot.
If it's happening consistently, it's worth reporting it as a bug directly at https://github.com/github/accessibility

yes