Visit the last post of a very long topic, then scroll up until you see the loading screen; this triggers the bug, including mobile and desktop.
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would there be an Uncaught: in promise error in the browser console
none 
very strange
We’re seeing the same issue on our forum - where if you take a longer topic and start to scroll upwards from the newer posts, eventually it will start jumping up by 20 posts at a time. This has been tested on various browsers, OSs and both mobile and desktop, and with the video from @Teafh it has the same behaviour. We’ve just updated this morning but this was noted a couple of days ago by some users.
do you experience the same behavior in safe mode? that would help rule out theme/plugin issues
Yes, the same in safe mode, both on Meta and my site.
Correct me if I’m wrong, but this sounds like something that’s been discussed here (see Related, below) which happens because topics are lazy-loaded. When you scroll down from the top, topics pre-load smoothly as you go. But if you’ve jumped to the end, then scroll back up, the earlier topics of various length start loading – and it just looks a bit squirrely as they push everything else down.