I've noticed this problem for the last couple of years as well and thought it was just me going too fast, being on the wrong layer, or just being ignorant of something. It is Dec 2020 and this is still a bug. The original post description from 1/2017 above is my exact problem and none of the fixes listed work for me. I have to shut down AD and reopen all the files to access the color gradient and have it work again. I am on an early 2015 MacBook Pro running Big Sur (but it was a problem with Catalina and High Sierra as well, but on the same machine).
