Getting adWillPresentFullScreenContent Callbacks But No Impressions

[DavidSurveyor-64]

My iOS app shows interstitial ads with AdMob. In testing with test ads, I get near 100% impression rate. In production, only 5% of presentation attempts result in billable impressions. I've added extensive tracking and found that interstitial.present() fires (as well as the adWillPresentFullScreenContent callback), but adDidRecordImpression rarely does in production.

Last Week's Numbers (Production):
- 692 present() calls (and adWillPresentFullScreenContent callbacks)
- Only 37 adDidRecordImpression callbacks (matches my admob dash)
- ~ 95% loss rate

A Few Notes:
Test ads work perfectly
I preload ads up to 30 seconds before showing (well under 60 min expiry)
I'm trying to make sure I get the topmost view controller
I've tried verifying ad freshness with some telemetry and code

Best thing I can come up with is...

Production ads have heavier creatives (video/rich media) that take longer to load than test ads. During this delay, something happens (app backgrounds? VC destroyed? Network drops?) that causes AdMob to not record the impression, even though the ad appears to present.

Any insights appreciated! How should I try troubleshooting? Anyone seen something similar??