This is known to happen occasionally for users who use Gmail in their browser, and a few other niche desktop email clients. It takes a split second for Senders to augment email messages after they arrive in your inbox. Sometimes, if you have interacted with the message while it’s being processed this may confused your email client about whether there are unread messages. Don’t worry; there is no risk of data loss. Try refreshing your mail client and see if that resolves the mismatch of read and unread messages.