Whatsapp contact mystery

I'd like to understand this behaviour in iPhone 11 latest iOS about Whatsapp contact:

I'm using only Gmail contacts and Gmail account is synchronised to contacts.

I can see correctly contact XXX both on iPhone and on Gmail site; this number included the area code.

In Whatsapp when I write new message I cannot browse XXX infact my chat shows only phone number.

It likes XXX contact doesn't belong to Whatsapp.

I solved in Whatsapp in this way: 

I added whatsapp contact to the existing contact XXX so I gave twice the same number on the same contact, but the name XXX was changed to YYY.

Can you explain what happened ?