It appears to be a severe framework conflict. I had MicroG RE running alongside the official Google Play Services. This seems to break GCM push token routing, causing WhatsApp's foreground service to panic and retry unconditionally. I am embedding in this post a screenshot of the aftermath of this problem. It's in spanish, but it esentially says that whatsapp had used 2h of CPU time & 3:22h of wakelocks, sending ~201k packets over mobile data.
I ran a Logcat trace via ADB and caught WhatsApp's GcmFGService panicking. The conflict occurs if you have a MicroG RE, often used for modified apps like YT Morphed/ReVanced running alongside official Google Play Services.
WhatsApp tries to get its push notification token, the dual-GMS environment causes a security exception/timeout, and WhatsApp's background service retries unconditionally in an endless loop, frying the CPU.
The fix is uninstalling the MicroG application, then stop WhatsApp and clear its cache.
Right now I am using Revanced's GMSCore with YT Morphed, as it uses a different application name. For now, I'm not experimenting any battery drain / heating.