Documentation: Image:addMask() default value is true/opaque instead of transparent[opaque])
Adds a mask to the image if it doesn’t already have one. If opaque is true, the image will be set to entirely opaque. Otherwise, or if not specified, the image will be completely transparent.

Acvtual behavior is that addMask() sets an opaque mask. addMask(false) can be used to add an transparent mask.

I would suggest updating the docs to reflect this, rather than change the behavior

Agreed, changing the existing behavior could cause problems. Thanks for catching this!