Hi,
Not sure if your problem has been resolved or not, but because I also suffered the same issue with getting these annoying and unwarranted notifications, I kept trying to get help from Apple advisors but was stumped with no resolution. And until recently, I was able to resolve this.
So in addition to toggling off the permission under the Noise & Haptics for headphone notifications & Reduce Noise Levels:
Try This—->>>> It’s under Accessibility!!!
Step 1—>Go to Settings
Step 2 —> Accessibility
Step 3 —> under Hearing —> select Audio/Visual —-> look for Headphone Notifications and toggle that off!!!!!!!!


