I did some more investigations and found a thing in the GMS2 release notes: (this post). Seems to be broken in runtime 2.1.4.218, worked in runtime 2.1.5.246, and it works for me in runtime 2.2.0.258... what's your GMS2 runtime version?
From what I gather there's an issue where valid GML becomes invalid C++/JavaScript code sometimes when a single GML statement becomes multiple instructions when parsed, which sounds like a nightmare to track down :/
(Since 2.0.3.56 there's also an option to carry on and collect all errors that's possible before ending a compilation, it might be helpful in finding the resource that causes this to happen... perhaps?)