new uGUI mask doesn't work for some android devices.

I have many devices, and on one of them, the mask does not work correctly.
Does someone have the same issue ?

This is on a Wiko Clink Slim (Android 4.1.1)
The English button does not use a mask, the rest does.
If you look closely you can see that rounded borders have the color of the Image behind.
View attachment 112904

This is on an Oppo Find5 (with android 4.4) and it work as expected
View attachment 112905