using in operator in custom fields does not work correctly

[macrozone]

i did this request:

const result: ListResult = await this.chargebeeApi.plan
      .list({
        limit: 100
      })
      .param({
        "status[is]": "active",
        "cf_productgroup_id[in]": productGroupIds
      })
      .request();

where cf_productgroup_id is a cutom field.

this returns all products, regardless whether cf_productgroup_id is set or not.

This seems very inconsistent and a bug