<proc>only</proc> does not work as expected

### Affected version

3.14.1

### Bug description

I attempted to create a separate Maven execution for the Log4j annotation processor as described in the [Log4j plugin documentation](https://logging.apache.org/log4j/2.12.x/manual/plugins.html).

When the second execution is bound to the `process-classes` phase, the plugin outputs
`Nothing to compile - all classes are up to date.` and the annotation processor does not run.

If I move the execution to the `generate-sources` phase, the processor runs correctly and generates the expected output. However, the log message says `Compiling n source files with javac` which is misleading because no source files are actually compiled—only the annotation processor is executed.

It appears that the maven-compiler-plugin does not handle the setting beyond passing it to javac. This makes it difficult to run annotation processors in isolation without triggering confusing or incorrect log messages.