Hello there!
Recently we've faced with situation, that field AdvertisingChannelSubType doesn't returned, while working via CampaignService. These field was always specified in our 'GET' requests and we didn't changed API version since 201708, it worked well till 4th quarter 2017. According to API docs, this field isn't marked as deprecated yet, so it's strange, that we no longer receive it in answers.
Regards, George.