Retained Accessory mode not working in 2010 Prius

It used to be that if I turned the car off, hit the Start button again to get into acc mode (all while in Park), the radio would stay on, with no interruption. Now I miss some seconds of radio while the acc mode activates. From what I have read, the feature that stopped working is called Retained Accessory Mode. Did I inactivate it somehow? If so, how do I get it back?

 

I have not noticed any symptoms of an unhealthy 12v battery, unless this is one. I need service soon, so I guess I can ask them to check that if they think it might be the problem. Thanks.