canvas empty in mobile devices

Hi, i have a canvas (red BG) and a yellow circle.
when i test it in my browser i can see both.
when i test it in my tablet (xampp server) i can see both.
when i test it in my smartphone (xampp server) i can only see the red bg and the contet disappears.

if i manually make an html with a graphic inside a canvas i can see both (xampp server).

WHY??