GraphQL filter with AND / NOT logic

[llamafilm]

I want to get a list of devices with a certain role and a non-null location. I tried the below query but the second part of the filter seems to be ignored. In other words, it returns all devices with role_id=15, including those with no location. What's wrong with this query?

query MyQuery {
  device_list(filters: {role_id: "15", AND: {NOT: {location_id: null}}}) {
    id
    description
    location {
      name
    }
  }
}

The below query works as expected to find all devices with a non-null location:

query MyQuery {
  device_list(filters: {NOT: {location_id: null}}) {
    id
    description
    location {
      name
    }
  }
}

[llamafilm]

To add more context: Ultimately what I'm trying to get is a list of locations which contain at least one device of that role, to be used as a query variable in Grafana with its Infinity data source plugin.