Does the volume actually increase ‘significantly’ on all the grouped speakers themselves? If not, it might be the case your mobile controller/App has momentarily lost its connection to one, or more, of the grouped speakers and their volume did not actually increase.
 

What type of network is it? Is it a centrally based router, a mesh system, or a wireless setup with multiple WiFi access points/extenders?

Yes, the volume increases significantly on all the grouped speakers themselves. It’s a 2.4 ghz wifi central router network. There are no range extenders involved, I know Sonos has said speakers do not work with range extenders. Otherwise, everything works great, no problems.

The weird thing is that if I increase volume on a group and leave it, the volume will stay as set. However, the minute I start moving the group volume slider up or down, the group with the higher volume will revert back to the level of the other groups. Seems like Sonos has some weird algorithm working in the background. Not a straightforward behavior for the sliders.

Once you’ve set each speakers volume within a group, if afterwards you decide to alter the ‘group volume’ slider control, then each of the individual rooms volume will adjust as a ‘percentage’ of the group adjustment - so perhaps the best thing for you to try, is set your individual speakers and if you don't want some rooms to change their volume more than intended, then simply just adjust the individual speakers volume instead and perhaps see if that resolves your issue.

That’s a good suggestion. I’ll just use individual speaker adjustments. That group volume adjustment slider is definitely wonky! It messes with the individual sliders in very unexpected and unintuitive ways.

Since the windows 11 upgrade does not allow me to adjust individual rooms from my desktop, the sonos tech said I could do this on my phone app while they are working out the bug. However in order to access the individual rooms on my phone, I have to hit the group volume button first. But the round icon is so sensitive that when I press it ---it changes the group volume up or down (frustrating). Additionally when I change the volume in one room, it mysterious changes the volume of other rooms. It’s like the app has a mind of it’s own. Potentially similar problem (I think) to pixelpaul. Any advice on the group volume sensitivity and the mysterious auto adjust?

Same reply to your duplicate post:

Same problem with adjusting volume or muting of individual speakers in a group. It’s apparently a bug in the S2 software, and I’m told that they are working on a fix.