Adsense Reporting – Finding Earning not Attributed to an Ad Unit

I’ve been a publisher using Adsense since 2006.

Up until sometime in 2020, I was pretty good with having all ad placements allocated to (or "tied to") Ad Units.

Recently though, as much as half of my actual earnings don’t show up in the Ad Units report. e.g. If my overall earnings on a given day was $ 60, only $ 30 of that shows up in the Ad Units report.

Clearly I must have an ad (or multiple ads) running that don’t have an Ad Unit?

How would I go about troubleshooting that?

It’s all on the same site/domain so hopefully it won’t be too hard to track down.