Not sure what ‘resets to zero’ means in this context, but clearly it is an issue. Have you submitted a system diagnostic within 10 minutes of experiencing this problem, and called Sonos Support to discuss it?  Don’t post the resulting diagnostic number here, they get sensitive about GDPR.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

Hi Bruce, by “reset to zero” I mean the EQ setting goes from 10 to 0 without me having changed it.

I do appreciate your feedback! 

Sub audio screen, Sub level setting?

Try powering down the sub for a minute, power it back up and make the setting. Let it sit for a few minutes and check that it is still set. Power the sub down, wait and back up then check it again. 

If that doesn't get it to stick try removing it from the room, power cycling and adding back.

Last ditch, remove it from the room, factory reset just the sub, then add it back to your system.

You can’t trust the EQ settings on the mobile app.

Use the desktop app to verify the actual behavior.

I can here looking for answers to the same problem. I have a new pair of Sonos Five and added to Sonos app today. Updated firmware and smoth sailing until I toggle the EQ to the right. Slider does not stick and I watched it slide back after about 5-10 seconds. Tried a few times and same results.

I restarted my S25 Ultra and try again. This time I slide the EQ without playing music. It worked as intended. So the hack is to adjust EQ without music playing. Was using a pair of Sonos Play 5 Gen 1 with S1 app. No EQ issues there.