The preheader of the email, which we send in the create email request, can be overridden by the preheader of the send definition. We don’t set that, but that “can sometimes happen if one manually clicks around on the emails”, according to Levementum. In that case it gets stuck, since we update the send definitions, not recreate them. This PR clears the preheader of the send definitions in case it gets magically set again.