Using two stereo-paired HomePod minis as a default speaker

Hello,

I am using Apple TV 2021 and two stereo-paired HomePod minis.

After the recent tvOS update, I set the minis as a default speaker of Apple TV, but only one mini works while the two minis are still stereo-paired - I checked it through Apple TV Settings app and iPhone Home app.

If the TV speaker is set as a default speaker and the paired minis are used as a temporary speaker, they work normally. the stereo-pairing doesn't seem to work normally only when those minis are set to Apple TV default speaker.

I would like to know why and how to solve this problem.

Thank you.