Looks like there is an encoding issue with that Drafts Template Action. There should be “icons” next to the [[deadline]] merge fields, but they’re showing as “?”
You should be able to still use that action as a reference, but use the actual “Thingy” action group to replace that “?” with the proper Deadline identifier.
EDIT:
Actually it looks like it’s an issue with the Thingy action group.
Anyway, here’s a slightly more recent write-up on how I process my Templates.