90% of my emails are marked as read before I even opened them. It has nothing to do with a “marked as read after x seconds”. Emails are randomly marked as read just after receiving it. It’s really confusing in full mailbox after weekend.
And no, I dont read them on another device. I’m using IMAP protocol.
There was also this thread, which is unsolved, but deals with conversation view. You could try Jay’s suggestion to select Show conversations in message detail only.