An interesting experiment would be the same thing you are doing but isolated in a note taking app to discard the google keyboard you’re using. Also, it would be interesting if you can use e.g.: proxyman (available directly on iOS), or some proxy on your PC to intercept your network traffic and then try to reproduce while blocking/allowing some domains. Especially, blocking all google domains, then facebook domains, etc. If you have a Pi-hole set-up doing that at dns level may be easier.

I’ve never been able to reproduce these experiments. But keep in mind that I’m european and my WhatsApp app is slightly different - it is a version from WhatsApp Ireland (instead of WhatsApp Inc) which shares less data with third parties, the privacy policy is also slightly different for the european union.

Edit. Another idea: try to reproduce while disabling predictive text on your keyboard.