I successfully applied a rainbow gradient to all the items on the first image below and exported some PNGs, etc and all looked good. I saved as .eps and .ai. I came back a while later and opened those same files and one shape out of the many was black and nothing I do can get it back to the gradient all the way across includingthe & symbol. I made zero changes to it from when I saved and had it applied correctly and when it wasn't working (no software/hardware changes either).
I am asking the original artist of the image too (I know just enough to be dangerous but not enough to figure this out), but wanted to ask here in case I don't hear back quickly.