You are using Case Merge Premium and notice that the HTML of an Email Message was cut off after being merged to the Master Case.

The standard HTML body field on the Email Message object is able to hold up to 32,000 characters (bytes) of HTML. While this limit is enforced within custom apex code, standard Email-to-Case will allow for Email Messages containing over 32,000 characters in the HTML Body field to be processed and saved in Salesforce without issue.

In the event an Email Message record containing over 32,000 characters in the HTML Body field is detected during a merge operation, Case Merge Premium will automatically truncate the email after 32,000 characters to avoid errors during merging.
 

This behavior is by design in the event Email Message copied during a merge operation contain more than 32,000 characters in the HTML Body field.