This seen with an IMAP4 account hosted by runbox.com Settings used are default apart from the server address, user name, password, etc. and SSL for IMAP and STARTTLS for SMTP mail sending. Spam messages are automatically moved to the trash folder by the runbox mail filter. They are marked as unread. Accessing my inbox using claws changes all mails in the trash folder to being marked as read. This even though I am not viewing the contents of the trash folder. OS version 5.2008.43-7
Platform is not supported anymore.