vim ignoring .vimrc syntax highlighting

I had a ~/.vimrc with some basic syntax highlighting that was working fine until I upgraded to Catalina. Running vim -V I see it loads my vimrc but it totally ignores anything in it. If I run :so ~/.vimrc inside vim it also ignores it, but if I run the commands from the vimrc one by one then it accepts them.

How do I get vim to source my vimrc again?

My syntax rules were being overwritten by later files in the load order, for some reason this happens now when it didn't before. The solution was to copy my .vimrc into ~/.vim/after/syntax/syncolor.vim (the filename matters too apparently)