QPushButton released() signal not always emitted when double buffering is disabled

Preconditions:
Disable double buffering in your application. This cannot be reproduced if double buffering is enabled.

Steps to reproduce:
1. Press mouse in QPushButton (or any item that inherits QAbstractButton)
2. Drag mouse to the edge of the button (it doesn't matter which direction). The cursor shouldn't go over the edge but to really close to the edge (check attached screenshot).
3. Release the button

Actual results:
4. released() signal is not emitted

Expected results:
4. released() signal is emitted

I attached an example application that can be used to reproduce the issue. When you press the button the value below of the button increases and when you release the button the value decreases. I.e. the value should be always 0.