Bluetooth LE : read or write in characteristic with security enable fail and randomy crash in step by step

When I enable security mode "Just Work" in my remote device, the Qt app on Windows in no longer able to read or write in my service.
The problem is only for "my" custom service (it is working when I try to read in services Generic Access, Generic Attribute and Device Information).
I have the same behaviour with the example app called "low energy scanner".
Note that I do not perform any kind of pairing / bonding / connection from the Windows settings. I wish to manage to discover a totally new BLE device and connect without external "software".

When not using Qt and using the android app nrf connect instead, this works. It also works if I use the Bluetooth LE Explorer Windows app.
It seems Qt app never sent the bonding request while nrf connect does.

Note : the service I'm using is defined as Unknow Service by the example app. Moreover this service is the Serial Port Service designed by u-blox compagny if this information can be relevant.

Looking deeper in debug step by step, I saw that app periodically crashes and discover I get the same message as this ticket https://bugreports.qt.io/browse/QTBUG-74758 which is closed. I think both issues are ble security related but not sure.