From the manual : Note the browser may ignore this request unless the action is in a user-initiated event, such as a mouse click, key press, touch event or button press.
I've read the manual, and there was (now it's gone) another line after the one you quote:
"(The one exception to this is NW.js which can request fullscreen at any time.)"
Is this no longer true? And, even stranger, if I put the same condition ("On start of layout -> Request fullscreen") on the fourth or fifth layout, it works fine...