The message has an HTML section, but doesn’t have anything in the HTML section, so if you are displaying messages in HTML, you won’t see anything. Thunderbird displays it exactly the same, and I looked at in Gmail webmail and Outlook on the web, and they do it the same as well.
It is an error by the sender’s email application. Either there shouldn’t be an HTML section because it really is a plain text message only, or they should follow standards and provide the formatted version of the content in the HTML section.
We will investigate if there is something we can do though. In the meantime, just right-click in the body of messages like this and choose Format > Plain Text.