Qt 5.11 uses a new Central role implementation on BlueZ versions 5.42+. When the first instance off an application successfully connects to the remote peripheral via QLEController::connectToDevice() and is subsequently killed with CTRL+C or similar signals the connection tot the peripheral remains open. It is managed by bluetoothd.
The next time QLEController::connectToDevice() is called, the connect never succeeds because the already open connection is not properly handled.
The cause of the problem is a missing connected() signal emission.