I have tested further and found that the volume level is not remembered from the last time in Bluetooth mode, or any other mode (SD, AUX or FM). Actually, the only time the volume level is remembered is if you are in any mode and switch to another mode before turning off the Soundcore mini. For instance:
Power up, stay in Bluetooth mode, adjust volume, power down, power up and volume level NOT remembered.
Power up, stay in Bluetooth mode, adjust volume, switch to any other mode, power down, power up and volume level IS remembered.
Power up, switch to any mode other than Bluetooth, adjust volume, power down, power up and volume level NOT remembered.
Power up, switch to any mode other than Bluetooth, adjust volume, switch to any other mode, power down, power up and volume level IS remembered.
So, powering up and going to any mode and adjusting the volume and powering down does not save the volume level. It is the action of adjusting the volume level, in any mode, then switching to any other mode before powering down that saves the volume level.