Try resetting the app under App Preferences and rebooting your phone.

Same for me.  The app is going nuts now. Volume is uncontrollable. iOS 14.6  This is the first time in 10 years sonos hasn’t worked well. Not happy at the moment.  Thinking of returning the roam. No power adapter as well? That’s just cheap. 
app reset didn’t fix it. 

App reset didn’t work. 

I’ve just tried this with two ‘Roams’ and two ‘Moves’ grouped in the App, using an iPhone XR with iOS 14.5.1 and that’s okay… maybe it’s the latest iOS 14.6 update. I will update my iPhone and see if that changes anything.

Well it appears it’s not the iOS update, or the Sonos App, after completing the iOS update to v14.6, all is still working okay here for me, so maybe there is some other software (perhaps?) running in the background or in memory on the iOS devices that’s causing the issue? 

My wife has an (updated) iPhone SE and the Sonos App ‘group volume’ is also working fine on her device too.

Volume bug craziness went away immediately for me when I un-plugged the Roam from a wired charger.  So I’ve isolated it to when the Roam is connected to a non official power adaptor.  It destabilized the entire Sonos network and introduced bizarre App volume slider behavior and grouped volume bugs. 

I wonder how many Roam issues are linked to the cost cutting decision not to include a simple power adapter? If related Sonos has risked a big reputation hit, alienating new users to the brand and countless returns all to save a couple bucks on an adapter.  Considering how picky the Roam is with power adapter needs this doesn’t seem like a good call.

All is good again in my Sonos world, hope this helps someone.

Sonos does offer the option of a Sonos power charger for the Roam. 

 I’m pleased you were able to track it down. I would hope Sonos is able to reproduce the issue locally, or look at the diagnostics you’ve submitted and called in about, so they can see what is going on. 

If not happy with the charger, I would bin it… it’s just not worth the risk.