Thanks for the suggestion.
As it turns out, it doesn't care about the syntax for HH_Status. That works with both dots and square brackets. But Manhole_Type inside curly brackets was the violation.
if ($feature["HH_Status"] == 0) {return "Proposed " + $feature.Manhole_Type}
Edit: For my purposes, this worked:
if ($feature["HH_Status"] == 0) {return "Proposed " + DomainName($feature,"Manhole_Type")}