Issue bonding Android devices

Hi all,

I have a server peripheral device and I need to use authenticated pairing with encryption and bonding requirement. I have a code in my device working fine with iOS devices (several HW versions with different OS versions). But with android devices, depending on the OS version, sometimes I have to write the pairing key (Android 4.4.4 and 6.0.1) but with Android 5.0.2 I can connect and bond without writing any key. It's like if the OS jumps over authenticated pairing situation. I don't understand why it is. Any idea?

Thanks in advance.