I have similar but opposite problem. When I publish to Tableau Public, my fonts which are black show up as faded grey. I changed to them to red and they are light red. This only happens in Chrome (both on desktop and Android). In IE the fonts show up as fully black or red.
It's quite annoying.
I have been struggling with this problem as well. It's not ideal, but my solution was to figure out the color that resulted from the 40% shading, then just set that as the color (with the shading set to 100%). I'm assuming that this problem results from the transparency setting for the background cascading onto the text.