At the moment when you create a conversion host via the ConversionHost#enable_queue method it is not creating a message properly. The db fixture needs to be updated. To replicate, create a conversion host via the ConversionHost#enable_queue method. Currently you will see a literal "{resource}" string in the output. The expected output is information about the resource.
https://github.com/ManageIQ/manageiq/pull/18535
Created conversion host via API . irb(main):002:0> Notification.all.map(&:options) => [{:op_name=>"enable", :op_arg=>"type=Host id=4"}, {:op_name=>"enable", :op_arg=>"type=Host id=3"}, {:op_name=>"enable", :op_arg=>"type=Host id=4"}, Verified in 5.11.0.11.20190625161125_392c61e