Unexpected disconnection with iOS devices

Hi to all,

i am developing a device with nrf52840 soc and soft device 14 and SDK 15. my device work well with Android, Windows, Google web ble but it experience unexpected disconnection event with iOS devices in this way that after roughly 2min link terminated by my device(i saw this in the sniffer and is visible in attached communication dump). 

you can see in above picture that after sometime(exactly after LL_PING_REQ and LL_PING_RSP) connection terminated by peripheral(NRF52840). i can not undrstand that what is the problem and is exist any relation with ping request and disconnection.

can any one give me a idea to solve this problem? i also attached some sniffer log.

thanks

IOS_dc.pcapng