Null wrapUpCode in analytics detail query - bug or limit?

Running a detail query for interactions and getting null for wrapUpCode even when agents set one. Using the Python SDK with this filter:

filter = {
 "type": "interaction",
 "statView": "detail",
 "filter": [
 {"statName": "wrapUpCode", "operator": "exists"}
 ]
}

Is this a known limitation for the analytics API or am I missing a parameter?