Antroid 5.1, WhatsApp special contacts ringtones not working

[AC Question]

I've assigned special ringtones for some contacts phone calls and incoming WhatsApp messages. The phone calls ones are working perfectly --so, no problem with them--, but for incoming WhatsApp messages what always sounds is the default ringtone, even though I have configured it correctly, contact by contact, from inside WhatsApp.

These WhatsApp special ringtones were working fine two days ago. They worked fine for some contacts only three days ago, and today they are not working at all: only default ringtone sounds for any incoming WhatsApp message.