Hi Stefan,
Thank you for reaching out to us. When you query for AdNetworkType1, you will segment your data into a report with a row for each campaign and network combinations, along with the statistical values split between them. When you don’t have zeroImpressions included, you won’t obtain results that have no impressions, but when you include zeroImpressions, you obtain all combinations of campaign and network. You can read here for more details.
A workaround for this is to for you to specify in the “Where” clause what AdNetworkType1 you’re looking for using the predicate values specified here. If you’re still encountering issues, could you please send me your report query?
Thank you,
Bryan, Google Ads API Team