I doubt that this is a fault.

Are the rooms the same size? Are you sitting at the same distance from each unit. Regardless of the setting does each room play loud enough?

When you say volume level - is it the volume level reported by the TV or the volume level looking at it in the app.   As long as works I don’t think there is any concern as Buzz said unless it won’t play loud enough.

The Playbar hooked up via IR the volume level shown on the TV was completely arbitrary and not to be relied on.  With HDMI-ARC you get an accurate level reported by the TV.

If this is very annoying, you can fuss with Volume Control of PLAYBAR by setting Maximum Volume.

Thank You! 

Remember the volume you see represented is just a number.