Using local mail storage (as it presents in mh folder storage format). First unread message in the list always is checked as read automatically. First launch of claws-mail or switching to local mail folder automatically checks first unread message in the list as read. No matter what sort in list of messages is used. Descending or ascending by date, theme, "From:" mail or other all the case. What is really important that cursor line automatically jumps to top unread message (as it is shown in the message list) and immediately changes it status to read. It is really annoying as I use several local folders and have to switch between them. After several switches it is confusing to get what was really read by me and what was checked automatically. As I know it is the case for previous versions of clasws-mail also. As I understand the problem is that cursor never save the last state in the list of messages whithin the folder.
You can configure what Claws does when entering a folder in Preferences/Summaries. What you want can be done with "Open message when selected" (Never) and "Set default selection when entering a folder (Last opened email)