I have my nextcloud contacts setup with DAVx⁵. It’s syncing correctly and shows up as expected in the contacts app.
However, in the Messages app and WhatsApp, I only see the phone numbers listed, even though the contacts are listed when I compose a new message.
First: That’s sounds very bad. Never install the most spying app in normal profile. If you really, really need Whatsapp, put it in shelter [HOWTO] Using WhatsApp under shelter
No you problem: Do the apps have access in your device settings/apps/ access to contacts ?