No longer able to AirPlay from iTunes to Homepod directly
Since you're saying this used to work the way you want it to, then perhaps resetting the HomePod back to its default settings will do the trick.

To fully reset it, follow these steps:


Unplug the HomePod
Wait at least five seconds and then plug it back in
Wait at least another five seconds
Press and hold the top of the HomePod with your finger
Keep your finger there even after the spinning light turns red
Continue to keep your finger there even after Siri announces that HomePod is about to reset
Finally you will hear three beeps to indicate that your HomePod has been reset
You can now take your finger off


Once your HomePod is reset you will need to set it up again from scratch.