Hello everyone. I consider myself fairly competent when it comes to my work with LaTeX, but as everyone who uses it knows, it is definetely strange sometimes. And right now I have a problem which just irks me in the wrong ways:

I use XeTeX and a number of custom fonts for my documents, but when writing mathematical formulas I found out that the font I set for Greek letters (Gentium Plus) is used for letter commands like \alpha, \Alpha, ... but not for commands like \sum, \prod. As you can see in this example here. You can see XeTeX defaults to Computer Modern.

I've looked all over the net for solutions but I can't find one. I'm using mathspec, amsmath, amsfonts, amssymb as my font related packages and I set my fonts like this:

\setmainfont{Charis SIL}
\setsansfont{Helvetica Now Display}
\setmonofont[Scale=MatchLowercase]{Fira Code Retina}
\setmathfont(Digits,Latin){Charis SIL}
\setmathfont(Greek){Gentium Plus}

I feel like I'm going insane not solving this problem. Should you require it, my entire LaTeX template is here on Github. Please help, thank you.