GM is working on a fix I found from a quick search. Released on april 27..
Customers may comment about any combination of the following Radio System Settings being lost, not retained, or returning to a default value after the ignition is turned off and the radio goes to sleep:
- Audio Tone Settings; including equalizer or EQ, Fade, Balance, or Sound Mode return to a neutral, zero, or default value
- Language setting defaults back to English from any other language
- Radio Maximum Startup Volume has changed leading to a complaint the volume is louder or not as loud as what it was during the previous drive cycle
The cause of this condition is unknown but appears to only occur with software version V135118. This software was built into 2021 model year vehicles listed but would not be present in a 2020 model year vehicle unless the radio was reprogrammed or replaced and programmed with the V135118 software version.