I have specified the $MessageContent to include CR's without using -BodyAsHTML since plain text will work fine. When I specify the -Subject either as a variable(like I have shown in the script) or a string quoted inline, I get a request created with extra text in the Subject line.
E.g. I run the script above and I get a new request in Support Center with the Subject of "=?us-ascii?Q?Test Subject?=". This does not happen when I send the email to any other destination.