I have found that the Win API function GradientFill is not working properly in the PB 15 beta. I think this may be the cause of reported issues with the Advanced GUI Library.
I have a sample app in both 10.5 and 15 flavors. If you run them side by side, you can see that color2 (right side on horizontal) is not the correct color. It seems to be showing as Fuchsia when it should be something else. If you choose Lime for color2, it shows Blue instead.
You can't attach zip or pbl files so I'll need info on where to upload it.