I have Xiaomi Redmi Note 9 with MIUI Global 12.5.1 Stable. My Whatsapp message notifications suddenly started to pop up without any sound. They just pop up on my screen. I tried removing restricted data usage for mobile SIM card 1, mobile SIM card 2 and Wi-Fi and it always turns "restrict mobile SIM 1 data usage" back on. I also turned off battery saving option for Whatsapp. Still no clue how to turn the notification sound on again.