After this commit:

https://github.com/qt/qtbase/commit/0c02f133f3daee146b805149e69bba8cee6727b2

Calling QApplication::quit() before QApplication::exec() quits instantly on [NSApp terminate:nil]; line, while in Qt 5.15.2 it was behaving as it is said in the docs:

"If a signal connected (non-queued) to this slot is emitted before control enters the main event loop (such as before "int main" calls exec()), the slot has no effect and the application never exits." (https://doc-snapshots.qt.io/qt6-dev/qcoreapplication.html#quit)

This may result in critical bugs if you're not lucky. For example, rendering the application non-launchable, if it calls quit() and expects to shut down in a predictable way. For me it introduced the worst bug in several years in the stable released on everybody version of the app