If the message preview is collapsed and you're reading mails in the external message window, only the initial message (that was doubleclicked to open the message viewer) gets marked read. The other ones (that are opened by pressing N in the message viewer) do not get marked read.
This is with claws configured to mark them as read "when selected for 0 seconds". The bug only appears with message preview collapsed.
To clarify: apparently the behaviour depends on how you open the next message. Using "Go to next unread message", the message gets marked as read. It doesn't get marked as read when using "Go to next message" though.