Why is my notification volume so much quieter than ringtone?

I am having an issue where my notification sounds (i.e., text message) are substantially more quiet and muffled than the call ringtone.

When I adjust the slider for Notification/Ringtone volume, the sample sound that plays IS very loud. However, when I choose a text notification sound, or actually receive a text message, the sound is very muffled and quiet.

I have enabled/disabled Attention Aware features, which has no effect on the problem.

I have disabled the Change With Buttons, but this also has no effect on the problem.

I do not have any app-specific notification settings.

This appears to be a software problem, since the sample sound that plays when I adjust the volume slider is very audible. Does Apple have any suggestions or planned updates to fix this issue?