Now that HTML can be used in Custom Rule emails (SMA 9.1.318), I applied the <pre> </pre> tag to the Ticket Summary field to prevent formatting from being discarded (this is recommended for a different field in the admin manual on pp. 249 & 251). Using this tag at least displays the contents of the field in a usable format in the email - in separate paragraphs, rather than all in one long string.

However, the first line of the first paragraph wraps shorter than the rest of the field, and the lines are spaced out quite a bit. See attached screenshots for examples. This example ticket was created and edited directly in the portal, not via email.