I’ve been getting this bug for a long time, not just the recent iOS and macOS, but hadn’t tried to recreate it on Meta before. I just did, and it happens here, so I thought I’d report it.
In Safari, both on my Mac and my iPhone (latest iOS and macOS versions on each, but as I say it’s been happening for months), if you start to reply to a long topic (it has to have more than a few replies already) - type something in the box but then hit “Discard”, instead of staying in the same place it jumps up lots of posts so you’re now looking at older messages.