Fail to fetch the iPhone's now playing via avrcp1.5

I'm a bluetooth embedded software developer. Recently I'm developing a car's headunit and meet a problem with apple's bluetooth.

My headunit uses AVRCP 1.5 to fetch the now playing songs list in an iPhone with command 0x71 via bluetooth. And the player in iPhone I used is iphone's local media player(Media). Everything works successfully on IOS11.2.6. The headunit can show the now playing songs in the iPhone.

However, when I updated the iphone with the lastest version IOS11.4.1, the headunit failed to fetch the playlist in the iPhone. The now playing folder I fetched via AVRCP is always empty. Does anyone know the reason or the differences of bluetooth or local media player between IOS11.2.6 and IOS11.4.1?

Any advice will be appreciated. Thanks.