The reason is thst I run Jacoco for coverage and since there are no sources for my plugins I only get coverage for the test classes.
At runtime, to produce the jacoco.exec file, Jacoco doesn't need the source. It uses the .class data only, with the debug information (like line numbers) to work well.
Later, when building reports from the jacoco.exec file, sources are needed, but the report step happens separately, not during the test execution.
So you don't need the sources during your tests. This is not an issue.