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.
I’ve recorded a video of me doing it on my Mac: