The following is what I have tried for Windows10.
(refer : https://forum.qt.io/topic/111756/maintenance-tool-mandatory-account-and-no-valid-license/3)

Delete File:

%APPDATA%\Qt
qtlicenses.ini
qtaccount.ini

I deleted it including the Qt directory, but the problem was not resolved :(

Checking other dir:

%USERPROFILE%.qt-license

Those directory didn't exist.

And no comes a reply to myself !

We figured out how to hack towards the behaviour we need.

1/ we add an ObjectiveC .mm file to the project, to call somme MacOS Cocoa's stuff

2/ At application AppB startup, from this ObjectiveC file we call

[NSApp hide:nil]

which "hides" the application, and reactivate our AppA.

Well... This turns into some flickering of the active application (because AppA looses for a few ms activation when AppB starts ; then AppB hides itself and AppA gets the active app again), but we can live with that !

Thanks

I made it work by removing this flag: Qt::WindowDoesNotAcceptFocus