Thank you for using Apple Support Communities.
I understand that you are having an issue with the volume on your iPhone where it appears to only adjust the ringer volume and not the application volume. This would seem to be an issue that I would start with a basic restart of the iPhone. This article will remind you how to restart, if necessary: Restart your iPhone, iPad, or iPod touch .
If you still have an issue after that, please try the steps in this support article: If an app unexpectedly quits, stops responding, or won’t open .
If that doesn't resolve the issue, then I have a few questions to try to further isolate the issue.
1. If you go to Settings > Sounds & Haptics and turn "Change with Buttons" to off, does this change what you are seeing?
2. If you change the volume, even if you see the bell icon, does the volume of the sound within that app change?
3. Is Google Maps the only app that this happens with or are there others?
4. What version of iOS is installed on your iPhone?
Let us know how it goes.
Best regards.