Mailbird does not have the ability to mark an email as read or delete them automatically. If your email is marked as read automatically, or deleted, this means that the email in your server is marked as read by some other email client. Mailbird is simply syncing to the server.
We cannot really tell you which of your email clients is doing this, but you can use a simple elimination process to find out which one is causing the issue.
Simply disable all your email clients except for Mailbird, including your mobile/tablet email client. Then try turning them on one by one. If the issue suddenly appear, you will now which email client is causing the issue.