@Dae-0 Just tried the same routine as you and the volume after playing the sample audio in the app was ok when I returned to music. The only difference was that the playback was through my helmet headset with the Sena connected to both my bike’s Bluetooth setup (Ducati) and my iPhone; each paired with separate Bluetooth channels of the Sena. That said, I do have a shortcut that forces the audio through to the helmet and not through the bike connection.
Very strange as you say.