Maven acting strange if I create a 'runtime' directory

[BurtHarris]

@sharwell I don't understand this, can you help explain?

For years, we've had this is tool/pom.xml:

        <testSourceDirectory>test</testSourceDirectory>
        <testResources>
            <testResource>
                <directory>test</directory>
            </testResource>
            <testResource>
                <directory>../runtime</directory>
            </testResource>
        </testResources>

I think testResource ../runtime has unintended consequences, but they were not triggered until I created a runtime folder. What is a testResource for?

Background

Expected contents of tool/target/test-classes folder:

I was doing some reorganization to untangle the build system. Anyway, I created a runtime/ directory that was a peer of tool/ and started having the build files (e.g. package.json) in that directory. This had an unexpected impact on the maven folder:

The first time (after clean) test generation works OK, but if I re-run test generation, it fails: