I happened to have the Security Notifications on for a while now. I see the message: "X's security code has changed." pretty often.
Under what circumstances does a new pair of encryption keys get generated?
One circumstance is when you put your sim card in a different phone. The new phone recognises that you already have a WhatsApp account, as it's tied to your phone number, but it doesn't have your private key, so it will generate a new pair and start exchanging the public part.
Maybe the very fact that you want to be notified of key change events got you marked as suspicious ;-)
I think when people swap sims in their device it triggers a key change
A new pair of keys are also generated when a device is wiped and restored from backup (in my experience on iOS).