I've set the "mark as read" delay to 0 seconds (immediately). I've now observed twice or thrice that after reading a mail, the message immediately above it in the message list also had been marked as read. I currently tried to find a way to reproduce it at will, but couldn't... Reproducible: Sometimes
I can now reproduce it: Prepare you IMAP inbox to contain two subsequent unread messages, eg. by sorting by date and sending yourself two messages. Move the lower message into another folder. The message below it will be selected for display after the moved message disappears, but the second unread message *above* will be marked as read although it has not been displayed.
Apparently has been fixed in the meantime, have not seen it any more for quite some time now.