Thanks! 
It seems to work now here with my smartphone, although I don’t really get this:
(from github)
Increasing the breakpoint and decreasing the padding to get full coverage. Only applies to very tiny devices (even smaller than iPhone SE)
I guess you’re referring to the 2nd version of iPhone SE (based on the iPhone 8), but the iPhone 7 (mine) has the very same resolution (viewport), and it seems to be fixed for it too. So the fix also applies to iPhone SE, I think.
Anyway, it is solved now. Thanks again.