Hello Antoniogemayel,
Thank you for using Apple Support Communities! I understand that WhatsApp contacts unexpectedly disappear from your iPhone X. They disappear again even after adding them to the Contacts app using Siri Suggestions. I would be happy to help point you towards a resolution.
I suggest starting with the guidance in the following support article from WhatsApp:
WhatsApp FAQ - Problems seeing contacts
I also suggest confirming what the default account is that contacts are added to on your iPhone. This will be the account that WhatsApp contacts will appear under when you add them with Siri suggestions. Contacts for that account would need to remain turned on under Passwords & Accounts in order for them to appear on your iPhone:
Manage and delete contacts on your iPhone, iPad, or iPod touch - Apple Support
Add and use contact information on iPhone - Apple Support
Cheers!