https://wccftech.com/android-pie-will-remember-the-individual-volume-of-every-bluetooth-device-connected/

Before the final release of Android Pie, we had an idea about some of the new features the OS might bring. Some of the changes were legitimately exciting, for example, Android Pie would prevent apps running in the background from accessing the microphone and camera. However, not all of the features made it to the final release. The one that did make it is the ability for Android to remember the volume of each Bluetooth peripheral you connect to your device.

The problem arises when you use multiple Bluetooth speakers/headphones. You’d want your speakers to play at a different volume than your headphones. Until now, Android couldn’t remember the volume of an individual device. Upon connecting a new peripheral, the volume was set to the same value as the device connected before it. The problem is especially bad when you left something at max volume.