Sorry for the delay in replying. I did use a custom version of Whatsapp but the change would work just as well on the standard version.
What you need to do is decompile the APK.
Look in \res\xml\preferences.xml
Search for android:ringtoneType="all"
Seems to me that this isn't a MIUI bug, it's an XML issue within the WhatsApp application.
I've attached a modified version that should work. You will need to uninstall any WhatsApp version you have before installing this one.
Let me know if it works..