Hello Choudaryadnan,
It seems this issue appeared recently, <1-2 months, likely 1-2 weeks ago. I'd like to let you know you're not the only one facing this issue. It affects me as well, and 6 users of the Reddit portal have reported same problematic experience (DigVisual8346, lucius_neo, RefrigeratorPerfect9, swiftpromo00, mjs1988, boredclaudius).
This also applies to Siri – when the issue occurs, it announces the caller in the same unidentified way.
{What's weird, I faced Siri announcing "WhatsApp caller" and then for the second call from the same person ~5 minutes later Siri announced the correct name of the person. However, this aspect maybe from a different bucket – the person's WhatsApp account was not added in their contact of my iOS Contacts app, so the iOS magic was applied to match them, and I believe there wasn't a Focus Mode activated during these calls}.
I'm afraid it is more related to WhatsApp than iOS – it seems likely it began not right after the iOS 17.4 release, and WhatsApp clearly uses iOS features (sort of these API methods) to work with Focus Modes (for example, in the app you see the "Silenced voice call – Focus mode" text at a service message at a chat screen). Or it may be a combination of bugs from the both parties…
Please make WhatsApp (Meta) and Apple aware of your frustration – send them a note:
https://www.whatsapp.com/contact/?subject=messenger
Feedback - iPhone - Apple (I believe even if this has nothing to do at the iOS side, Apple people may have channels to help WhatsApp people become informed of the issue).
The more voices speak, the more quickly the message will be heard – and the bug fixed.
P.S.: 2 or 3 WhatsApp updates later from my initial encounter with this issue, today in version 2.24.10.74 it still persists…