What might be inflicting this sound auto-adjustment in a recreation?

I not too long ago received a brand new cellphone, S25, and redownload this morse code apply app MorseMania.

Within the video, you possibly can hear that the sound of the tone begins at one quantity and instantly drops to be quieter. This occurs actually quick, like 0.2 seconds. It drives me nuts as I attempt to apply.

I've adjusted the sport booster settings, and Dolby Atmos isn't enabled.

Thanks!