In the transform process the passedThough context param is used to track both mixins and build vars for cyclic loops.
The issue causes unintentional diagnostic messages when getting to a cyclic var through a mixin.

playground example - the issue is with the cyclic message contain the mixin tracking:

→ mix from /src/app.st.css 
↪ /src/app.st.css: b 
↪ /src/app.st.css: a 
↻ /src/app.st.css: b

Maybe knowing that the error came through a mixin is not that bad, but this wasn't intended to look like this and the formatted arrows don't look good, maybe indent the cyclic vars section.