Actually I tried it fast and after setting some qDebug it seems I never received the touch events.
I did what is suggested in this answer : https://stackoverflow.com/a/34740261
And now I am able to get the touch events and add "QApplication::processEvents();"
But there is no difference the problem is still here.
With this debugging, I saw that touch event are received even when the application is in the buggued state (black screen). They seem processed as usual, because when I hit Ctrl+Alt+Suppr, and my Window reappears, I see the result of the touch event in the GUI.
Some more investigation on the Windows side showed some incompatibility with the Intel Graphics Driver. The tablet is equipped with an Intel HD Graphics 620.
The app works with the driver Intel Graphics Driver 23.20.16.4973, which is a "DC" driver, now called "Legacy"
The bug appears with the Intel Graphics Driver 26.20.100.7637 which is a "DCH" driver introduced with the "Universal Window Platform"
https://www.intel.com/content/www/us/en/support/articles/000031572/programs/intel-corporation.html
Installing the "Legacy" driver on the Windows 10 Iot Tablet fixes the problem. But this is not a really good fix, at it implies to keep this driver version and does not permit updates.
Are they specific known issues with QML and Intel Graphics Driver DCH ?