Why do some of my contacts not sync with Google?

When I add a new contact on my iPhone then after a few seconds I see it in my Google contacts. When I remove it from Google contacts, then within few seconds it's removed from my phone. So far so good. However, I have a few contacts that I must have added on my phone in some strange way (maybe through another app like WhatsApp) that do not get synced. They are on my phone, but I do not find them in my Google contacts. Does anyone experience similar? **How can I fix this issue? Is there a way to force "all contacts sync" from my phone to Google?** device: iPhone X iOS version: 12.3.1 Default account : Gmail with mail, contacts and calendar sync ON, *no* other account with contacts enabled; sync method : automatic fetch