I think i might have the same issue.
I want to use Str::template($page->myWriterField(), ['mail'=>'some@mail.com']) in an email template where i create a link that lets the user unsubscribe from a service again.
When i enter the link in the writer field for the email reply i’d expect to be able to enter mydomain.de/newsletter/unsubscribe/{{mail}} but that gets sanitized ( { becomes %7B) and therefore the replacement doesnt work. I tried to use the custom text option for links, but that didnt help…