@B-iggy and I did some digging into this via PM and managed to track down the issue. The topic in question had some old tags attached, but tagging was disabled on the site. Those two things combined led to a weird edge condition in core which was causing the error. That particular issue should be resolved by
https://github.com/discourse/discourse/commit/3e8b6c67ea81e1f2824225d36c090592f9b8b36f
It’s possible there are still other issues, so I’m gonna leave this topic open for a couple more weeks. Please shout if you’re still seeing problems loading revisions.