iPhone automatically lowers the volume of certain audio sources when multiple audio sources playing

I originally had a GameObject that looped BGM, but when I dynamically added several AudioSources to the Scene to play battle sound effects, I noticed that the volume of the BGM decreased. After the battle sound effects finished playing, the volume seemed to gradually increase again, but very slowly. This problem only occurs on iPhone, and it seems that it does not occur in the editor and on Android. The inconsistent performance across different platforms is very frustrating. Does anyone know why this is happening, and how to fix it?