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[SOLUTION FOUND]

Thanks a lot for the answer. I started checking out what you suggested and everything has been resolved disabling WebGL. I'm so happy it was not one of those mysterious gimmicks that GM sometimes gives.
It's strange thought, as I thought WebGL'd taking care of most of the compatibility problems during the export instead of complicating them (I also had some graphic glitches that are now gone).

Just to give more infos:

• I was using Chrome, but I've tested it also on Edge and the result was the same.
• I was not calling the function in a draw event but in a global mouse button pressed.
• The application surface was enabled. The canvas was prepared to be saved as a screenshot thanks to a previous sequential draw procedure.