looks like checkstyle was waiting on compilation to finish to run, is that something I've maybe done? or is that just how gradle has set it up

It needs the classes: https://github.com/gradle/gradle/blob/614c318451d601d305873b099007d847aaeee42c/platforms/jvm/code-quality/src/main/groovy/org/gradle/api/plugins/quality/CheckstylePlugin.java#L138

but why... checkstyle itself doesn't need classes, it's a single file source processor only

🤷‍♂️

bug?

Possible

existing bug