Overview
Google Analytics's Engagement Time is an estimated measurement of a user's active time on a page. Sometimes, this value changes dramatically when a publisher changes ad providers. This does not indicate a problem with the ad implementation; instead, it typically indicates a change in the GA implementation.
GA calculates this value based on the events it records. As a result, changing which events it measures can change what the value measures—more information here.
Using the Google Analytics Debugger
The Google Analytics Debugger extension adds console logs for each GA event, helping you understand exactly what it records.
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Navigate to the page you wish to debug and click on the Debugger icon in the extension tray. The icon will change to indicate it is active, and your page will refresh automatically.
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When opening the Console, you'll see logs for Google Analytics events. Filter for_et. This will show you events that are tracking engagement time. In the image below, we can see an event occurring when the page is 60 seconds old and a record of how long the user was engaged with the site. -
Compare these events with the previous setup and current and note any significant differences. Also, run tests with an active ad blocker to see which events are being tracked when ads are not serving.