re-pairing failure with android device.

[JoonDong]

I tested gatt_security_server example with nRF connect android application.
First pairing and bonding succeeded, but re-pairing fails with following message.
E (47736) BT: gatts_write_attr_perm_check - GATT_INSUF_ENCRYPTION
I did not revise anything in the example and tested with both jelly bean and nougat.
And it works well with nRF connect iOS application.
Is this a bug?