I wanted to make use of https://issues.apache.org/jira/browse/MCOMPILER-391 but since I'm using ECJ (and lombok and MapStruct), I had all my annotation processors defined as maven-compiler-plugin dependencies as advertised here, not via annotationProcessorPaths.
So with the new compiler-plugin version I tried moving hibernate-jpamodelgen (for which I'm getting a depMgmt entry/version via Quarkus BOM) to annotationProcessorPaths and it just didn't work.

That's when I stumbled upon https://bugs.eclipse.org/bugs/show_bug.cgi?id=573833 and in fact, plexus-compiler-eclipse is passing -processorpath before -classpath.
Changing the order in EclipseJavaCompiler does indeed fix/work around this issue.

I'll come up with a PR in the next few days.