Running iOS 14.8 on 2020 iPhone SE but experienced this in prior versions too. When I'm using the Mail app to read messages in threaded mode, ie. where the messages show up grouped by conversation, the roter action to move through the messages appears buggy. Say I'm at message 1 of 8. I activate the roter option for messages and swipe down. It will go, for example, to message 2, message 3 but then jump back to message 1. This behavior of focus randomly jumping back to the first message in the thread happens repeatedly, making it almost impossible to reliably scroll through messages.
I hope this explanation of the issue was clear. Has anyone else noticed this?