Finally I got answer from Apple:
Indeed, there is a power-saving measure in CoreMIDI that will disconnect BLE MIDI connections if all MIDI clients have quit. Without BLE MIDI connections, this timeout is only five seconds. However, since we assume that some customers may open Audio MIDI Setup, establish a BLE MIDI connection, close AMS, and then open a DAW of choice, we wanted to prevent the connection from being severed while in transition.
MIDIServer is a launch-on-demand process, and we try aggressively to quit the process when it isn’t needed by MIDI clients.