Inbound Email Message for Email-to-Case Appears as Blank on Case Feed

This is working as designed, Case feed can only render HTML formatting. Salesforce doesn’t have control over how the sender sends the email message. If Salesforce received the email message as a text version only, Salesforce only populates the Text field of the email message record and not the HTML field. Since the Case Feed can only render HTML formatting, the email message displays as blank in the Case feed because the HTML field of the email message record is not populated. Anything that would prevent the Case feed from being able to process the HTML body of an email (S/MIME encryption, for example) would also run into this limitation

This limitation can also impact the rendering of Email Message records in other components, such as the Email related list on Case records, which will also render as blank.

See Also:
Send and Receive Emails with Email-to-Case
(Idea) Support S/MIME Encryption