make sure you're actually controlling your phone, and not the HomePod, Controls for both look exactly the same, and are accessed the same way.
The only difference on the now playing widget is right above the title of what's playing you'll see what you're controlling. You should see "iPhone" or "Living Room" (or whatever room your HomePod is assigned to). If you're in the Apple Music app, at the bottom center right below the airplay icon it will have what you're controlling. for your phone, it will be blank, for other devices, it will say what it is.
For apple's apps, most of the time it will remember the last one you used, so if you don't pay attention, it's easy to play music in the wrong place.
from the airplay screen you want to select "control other speakers & TVs" at the bottom of the screen, then select your iPhone.