I have also beginning this days receiving E-Mails with "High CPM from your device". I can fix this problem on the GMC website on my own user profile. The user profile there is a setting for set the private threshold. It was set to 30 CPM. exactly the value of the e-mail content.
Setting a private threshold on the GMC user profile sounds like a plausible solution. Aha, the threshold was suspiciously set to the exact CPM mentioned in the email! I recall once, troubleshooting a sluggish website and discovering a hidden script hogging resources; solving it felt like winning in Slope Game, a great relief. https://slopegamerun.com