Never really used DND 

You tried restarting?

Just googled, it's a pre-existing bug since MIUI V4.5.30 build 
SOS

Like always, lets hope the developers would fix it when lots of feedback are given

Encounter this before, but not all the time.

before upgrade to current latest whatsapp version i encounter this but only once and now seems ok. but sometimes i realise got vibrate but no sounds. 

Just a suggestion, maybe can try MY ROM or International ROM instead?

Definitely can.

But you need to:
1. Install CWM in recovery & Full Wipe
2. Download previous Full ROM version HERE or can use other ROMs
3. Install ROM in CWM
4. Full Wipe

Did the above for my friend's 64GB China Mi3 

Ops..
My bad..

Too distracted, smelling Ayamas Ayam Percik in the office 



Just uninstall, then download previous versions like HERE



Edit: Just checked, if update from Mi Market, latest version of whatsapp is 2.11.304 

Just manually backup:
Whatsapp - Settings - Chat Settings - click "backup conversation"

So when u reinstalled, it will prompt if you want to restore

Something to do with DNS flushing or what, can't remember.
But at least it's solved
For 1 & 4, try use hardware diagnosis to check,  *#, #64663#*#*
I think can calibrate as well.

For 3, high usage is good, fully utilize, worry when 100% usage & phone lagging even when in performance mode

I also receive the notification. Maybe update later as my phone batt is less than half now.

Whatsapp still coming in [unless u turn off wifi/data], only call and sms will be muted.