Hey!
I noticed a lot of topics with accounts being suspended lately, and the same thing happened to me.
For me the reason is that Google's Cloud Test Lab (release just a few weeks ago and still in beta) does not handle Ads properly.
When these tests run, the ads are being clicked randomly, which leads to the "invalid activity".
I understand that we can publish a different flavour of the app with no ads, or maybe even the "debug" version, but that defeats the purpose of promoting that same version to production.
I already sent an email via Google's Developer Console but I figured that since there are quite a lot of people affected by this I should give a heads up.
It would be great if either the Admob or the Cloud Test Lab teams could provide some feedback on how to proceed until someone sorts this out, and what will happen to the accounts that have already been suspended.