for me it honored HomePod’s right-left settings i configured in Home.app for them (confirmed with a right-left test track), so i assume that’s the default
if you have different results, you can use Roon’s Procedural-EQ DSP Filter to force stereo behavior (AirPlay 2 Support in Roon for Paired HomePods - #23 by Craig_Colthorp); that’s how i had mine configured until i found out (form a forum post here that i can’t find yet) that the DSP trick is no longer required with recent iOS updates