How did the WhatsApp support team knows I have a rooted phone?
From your build version OnePlus2-userdebug 7.1.2 NJH47F b4eb8634d4 , one can easily say that you are using a rooted build. 


  userdebug like "user" but with root access and debuggability; preferred for debugging


For more info about build type see this

In this case any root hide app will not work. You can try to edit build name along with root hide.