Can you attach the paint preset you are working with. Hard to fully comment otherwise.
I will say that i've worked with alpha channels in png images before without any problems in V5.
here is a video illustrating the problem
i just used Acrylic Blend, first preset from Factory>Manual Painting>General Examples
however i am not painting anything with this yet, just clearing the canvas to source image. the png and tif files are attached to the original post.