Iphone decreases it's background music volume

So I'm facing a problem that the music reduces to extremely low level or sometimes just lower on my XS once I unlock my phone by face id(not talking about the phone ringer). It doesn’t happen every time I unlock my phone after many attempts I encounter the reduction in music problem and this only happens in low battery percentage like below 30 is anyone else facing this issue? and I have also Attention-Aware Features turned off.