After opening the Pinned Message window for a channel on discord, all of the pinned messages are then prepended above all the currently loaded messages for that channel.
This also affects the message id/timestamp for retrieving messages further in the past, as the new reference message is now the oldest pinned message for that channel.

https://slack-files.com/TAQN6JAJ1-F017PALH4A3-c863994f13

The part where subsequent fetches start from the wrong point in history is fixed in the next release. The part where pinned messages show up in the main message log is not yet fixed.