Fallback session management causes Qt apps exit early on GNOME

Steps to reproduce:

1. Within GNOME 3.x session (tested on GNOME 3.36), launch any Qt application with fallback session management enabled (default),
2. In GNOME Shell, launch gnome-session-quit by any means: from terminal, GUI or shortcut,
3. Select "Cancel".

Expected behavior:

The application will do nothing visible by the user (although it may perform some session management, as described in the XSMP Protocol).

Actual behavior:

The application will either exit or prompt the user if it has such a feature. In other words, it'll react in the exact same way as if the user had asked it to close.

Example applications affected:

• Qt Creator 4.11.2
• Telegram-desktop 2.0.1
• Wireshark 3.2.3
• QCAD 3.24.3
• FreeCAD 0.18
• CuteCOM 0.51.0
• VLC 3.0.9
• basically all non-KDE (which seems to have worked around this)

Motivation

This issue was mentioned in QTBUG-49667, and was considered fixed since.
Despite that, it seems very impractical to me to ask every developer to implement a fix for their application, especially if it will be limited to simply calling QGuiApplication::setFallbackSessionManagementEnabled(false);. In my opinion, the default behavior must NOT cause application exit before the system actually is going to shut down.