File watcher compiling CSS into nonsense Chinese characters

I'm having the strangest error when compiling a CSS file from SCSS, via File Watchers.

If I clean the solution and build from scratch, compilation is absolutely fine. But if I make any change in the source SCSS, it recompiles into a collection of Chinese characters that don't mean anything.

If I have the file open when it refreshes, I see it happening.

Sometimes I have had a wrong encoding error beforehand, but not consistently.

Has anyone else had a similar problem? Or can help shed light on it?