The Business Rule in Journal.Email to calculate the field HasAttachments value does not update the field when inserting an Email. This can occur if the attachments are saved after the email, so in that case this rule would work as expected if the email is updated. This article describes a configuration that solves this issue in all the scenarios.
Please modify the "JournalEmailContainsAttachment" relationship as in the screenshot, so as soon an attachment is linked to an Email the field will be updated:
Please be aware that if your tenant's server email configuration has enabled the option "Keep full incoming message as an attachment", all the incoming emails would have an attachment.