OMG, thank you so much! You saved me from the insanity!!!
I have spent the last 4 hours installing android USB drivers on 2 laptops, configuring USB debugging, collecting logs...
I knew the solution would be something very simple and very stupid and was right.
I tried "On any touch start", "Is in touch" and "Is touching object", but my phone needed "On touch end"!
Oddly, requesting fullscreen after "On any touch start" works in other browsers, in Scirra Arcade, and apparently on other phones, but not on mine.
Phew... I can sleep now.
Thank you again!
NN81, big thanks to you too!