If elements are sorted the "message" element is not last and things like "reply-to" and "subject" are just included in the message body. If I take the monittrc written by the puppet module and rearrange the the elements in message-format so "message" is last and then restart monit it works as expected.
https://github.com/echoes-tech/puppet-monit/blob/ae2eb12c102169b1cbe383e7ec31270ab2f96808/templates/monitrc.erb#L17
If elements are sorted the "message" element is not last and things like "reply-to" and "subject" are just included in the message body. If I take the monittrc written by the puppet module and rearrange the the elements in message-format so "message" is last and then restart monit it works as expected.