DND not silencing notification sounds


May 18, 2019
Mi 9 on latest beta build 20.12.9.
I've never used DND before. I've started using it now to avoid vibration during the night on notifications.
But for some reason, at least WhatsApp private messages still sound as normal. It doesn't vibrate. But it sounds. I think it doesn't happen with group messages, and not with some other notifications. But I think there's some bug or something with the DND mode that's not working.
Help to debug the problem is appreciated.