plain text in mail differs from html text when sending multiple mails at once

When I activate multiple users at once, the html version of the mail looks perfect, but the text version in the multipart mail shows the wrong username.
( The mailtemplate comes from wp-members, one of the shortcodes is [username] )
If I change WP Email Content Type to HTML the problem is gone, but I would like to send it in multipart for those who can’t read the html version.

WP Email Template LITE Versie: 2.1.0

Thanks!