Thank you for reporting this issue. I figured out that the issue only occurs on PieFed.

The root cause is that Lemmy uses the same endpoint for getting all private messages vs privates messages between you and someone else. PieFed uses two separate endpoints. Summit only uses the general private message endpoint so PieFed just ignored the specific user argument and just returned all messages which is why no matter which conversation you tapped on you always saw the full list of messages. Fix is to split the call into two for PieFed only.

This bug will be fixed in the next release.