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It appears you’ve encountered a fairly unusual issue with the Google Analytics Reporting API. After thoroughly checking your request and dates, it’s quite strange for userEngagementDuration to produce inconsistent results, especially isolated to pairing with the medium dimension. As you mentioned, there is no sampling level issue with BatchRunReportRequest to attribute to this problem.
There are few things you could potentially explore:
1. Dimensions and metrics can sometimes behave unexpectedly when combined. It may help to verify all metrics and dimensions on the Google Developers metrics & dimensions explorer page and see if there may be any known quirks or compatibility issues when userEngagementDuration is paired with medium dimension.
2. Cross-check to confirm any filters involved are working as intended without inadvertent exclusions or inclusions which can affect results.
3. It is also worth checking your API version as Google occasionally updates these and changes may not always be backward compatible.
You should also consider reaching out to Google Development Support as they may be able to provide more specific guidance, or it may be a potential bug that they might need to look into.
