I'm running the same request to CAMPAIGN_PERFORMANCE_REPORT and AD_PERFORMANCE_REPORT, with the exception that the latter includes the ad ID field. With CAMPAIGN_PERFORMANCE_REPORT, we get back plenty of data, but the response for AD_PERFORMANCE_REPORT contains just the headers.
Requests:
CAMPAIGN_PERFORMANCE_REPORT: url:
https://adwords.google.com/api/adwords/reportdownload/v201809 post: '__rdquery=SELECT+Date%2C+AdNetworkType1%2C+CampaignName%2C+CampaignId%2C+Cost+FROM+CAMPAIGN_PERFORMANCE_REPORT+DURING+20190114%2C20190120&__fmt=TSV'
AD_PERFORMANCE_REPORT: url:
https://adwords.google.com/api/adwords/reportdownload/v201809 post: '__rdquery=SELECT+Date%2C+AdNetworkType1%2C+CampaignName%2C+CampaignId%2C+Id%2C+Cost+FROM+AD_PERFORMANCE_REPORT+DURING+20190114%2C20190120&__fmt=TSV'
For the latter, the response is just "Day Network Campaign Campaign ID Ad ID Cost "