This does not explain my issue which is IDENTICAL to the other poster replying to your post.
I have an iPhone 8 Plus running iOS 13.4.1 and this issue is new since the last update or possibly the update before.
My issues is that the volume (in any app using sound) decreases when I receive any notification but the volume does NOT return to the previous volume level after the notification is timed out & screen goes black. Even when I physically dismiss the notifications or click thru to the app from the notification there is no change. I have to use the volume on the iPhone or headphone buttons (multiple make/made used).
I don’t believe the IPhone 8 has face recognition and therefore doesn’t have the Attention Aware feature so your explanation doesn’t explain my issue.
Sorry this was so long but I’m very frustrated and I didn’t want to leave anything out.