I came back to the forums to report a similar problem but I might have figured it out. My text and Signal notifications were no longer consistently causing audible alerts. Different apps. At first, I couldn't figure it out. I think I may have found the issue.
When I adjust volume from the home screen, it's adjusting MEDIA volume, even though no media is playing. I could swear I opened up all volumes several times and jacked the notification volume all the way up. I also seem to recall a setting where you pick the default for the volume button but I'm not sure and I don't see the setting in the volumes screen for that.
This was absolutely not an issue until a couple days ago. I wonder if the Nov update did something.