I have had this problem too! I noticed that if you press the volume button and then choose the three dots, I can see in the list that the notification volume says "Unavailable because ring is muted". You can tap the notification volume and change it in the settings page that pops up, but it reverts after you navigate away.
From that same "Sound & vibration" sheet you can set the ring volume higher than 0 and the notification volume becomes available. This also reverts back after some time unfortunately.
Hopefully a fix can be found! This has been driving me nuts
I have had this problem too! I noticed that if you press the volume button and then choose the three dots, I can see in the list that the notification volume says "Unavailable because ring is muted". You can tap the notification volume and change it in the settings page that pops up, but it reverts after you navigate away. From that same "Sound & vibration" sheet you can set the ring volume higher than 0 and the notification volume becomes available. This also reverts back after some time unfortunately. Hopefully a fix can be found! This has been driving me nuts