Haha see? If you read carefully there's post above that if you disable MIUI optimization the settings will be reset.you need to sync your Google account again so your contacts will link to WhatsApp. So simplethen everything will be ok.
sometime i just wonder why they make their FW so unfriendly for user just to enable a very simple and common notification that many user will need?
they really so kia su to optimize every bit to save battery so can claim their phone has the most lasting battery life?

