ANCS problem in SDK15.0 of disconnect by the reason of 0x13

Hi, nordic

      I get some problem when developing the ancs with SDK15.0.

I run the demo with  ble_app_ancs_c on the board of PCA10040.

First of all, after my iphoneX (system version 11.3.1) connected to the ble device, with bond,the ancs works well.

and then I turn off the ble of my phone and turn on to reconnect to the ble device.

After the reconnection, the ble device is disconnected about every 30 second which the disconnect reason is 0x13( REMOTE USER TERMINATED CONNECTION

This really confuse my deeply cause I have do nothing modify to the demo in the SDK .

can you help my to solve this problem?

Best regard!!!