After the latest Apple iOS update, it's very common for subscribers' mailboxes to open emails automatically.
Unfortunately, this isn't anything senders can control but is controlled by the subscribers' mailboxes.
You can learn more about this here: https://convertkit.com/ios15-open-rate29. There's no way to tell if a subscriber's email is being opened automatically or if they actually opened it themselves.
This can happen to any email address: gmail.com17, yahoo.com21, icloud.com26, custom addresses, etc.
If you have further questions, I recommend you email at firstname.lastname@example.org to take a look at your account.