Apple Music airplay difficulties, Mac Pro ventura 13.7.4, HomePod mini

I routinely airplay from my Mac to a HomePod mini, with audio from both my desktop speakers and the HomePod concurrently. been doing this for years.

recently, there have been issues. whenever I select both the desktop speakers (connected to headphone jack) and the HomePod for output, the local speakers cut out after a short bit, with only the HomePod producing output. the airplay panel shows both the local speakers and the HomePod are selected for output, as I requested.

if I de-select the HomePod, the local speakers come back on instantly. and the HomePod goes silent.

I've always been able to play audio to multiple outputs, until now. it's either one or the other.

any ideas? thanks!