In the bluez implementation of Qt Bluetooth Low Energy, a socket is used to send messages to the remote device. The socket is created without the Unbuffered flag, so multiple subsequent writes can get coalesced into one message (stream behaviour). This behaviour causes problems when the remote device implements a message-based protocol (like GATT).

An attempt to write two BTLE characteristics subsequently will reproduce the problem.

Forcing the socket to Unbuffered mode solves the issue (like in this, seemingly unrelated Android patch: https://codereview.qt-project.org/#/c/96779/3/src/bluetooth/qbluetoothsocket_android.cpp)