I’ve reported this to Airmail support and apparently it’s a known issue. This known issue has been around for a long time:
If an incoming e-mail is in HTML format, the preview window truncates the e-mail. You literally cannot view the email beyond a certain length. The workaround is to view another e-mail and come back to it at which point the preview will display the message in its entirety.
It’s not a showstopper but it’s EXTREMELY annoying not to be able to read the message upon its first selection. It is a bit of a UX killer.
Fivetwelvish about Airmail - Lightning Fast Email, v2.6.1