UE_REG_ADMIN_PA_SUB not processing substitutions

In the subject line for admin email notification of new users requiring moderator approval (UE_REG_ADMIN_PA_SUB), I added [USERNAME] to this string as a language override, but when mailed it does not replace with the actual username, it only shows as [USERNAME] . In the body of the message (UE_REG_ADMIN_PA_MSG), however, [USERNAME] is replaced with the actual username. How can I get the username to show in the subject line as well?

Subject is passed through substitutions so should work fine to use [username]. Beyond that I've no idea what to suggest. It's passed through the same substitution function that the email body is.