Hey Christoph Berg, I looked into this further and saw that DefaultMatrixExecutionStrategy is setting the build result by using Result#combine, which "combines two Results and returns the worse one", where "worseness" is defined by a simple ordinal system in Jenkins core:
public static final @Nonnull Result SUCCESS = new Result("SUCCESS",BallColor.BLUE,0,true);
public static final @Nonnull Result UNSTABLE = new Result("UNSTABLE",BallColor.YELLOW,1,true);
public static final @Nonnull Result FAILURE = new Result("FAILURE",BallColor.RED,2,true);
public static final @Nonnull Result NOT_BUILT = new Result("NOT_BUILT",BallColor.NOTBUILT,3,false);
public static final @Nonnull Result ABORTED = new Result("ABORTED",BallColor.ABORTED,4,false);
Based on the semantics of Result#isWorseThan, NOT_BUILT is considered "worse than" SUCCESS, hence the behavior you are experiencing. A similar problem exists in workflow-cps plugin when using the parallel step without failFast. As of workflow-cps-plugin#325, the parallel step propagates the "worst" result to the overall build (just like with the Matrix Project plugin), but it uses the same definition of worseness provided by Jenkins core and hence has the same behavior you are seeing here.
Coincidentally, I was the one who implemented workflow-cps-plugin#325, and I noticed this annoying behavior while writing the tests for that change. I proposed a workaround in workflow-cps-plugin. Maintainer Devin Nusbaum responded as follows:
To elaborate on my earlier comment, the values in Result have always seemed weird to me. ABORTED and NOT_BUILT feel like a different kind of thing entirely than SUCCESS, FAILURE, and UNSTABLE. Sometimes I think it would have made more sense for results to have 2 parts, a "completion" result, like , NOT_BUILT, ABORTED, COMPLETED, and a "goodness" result, like SUCCESS, WARNING, ERROR. Then you could have something like a Pipeline step that is ABORTED and SUCCESS in the case of a user cancelling the build, or ABORTED and WARNING in the case of something like a timeout, NOT_BUILT and SUCCESS could be used in cases like a Declarative when expression that is false, and here in the parallel step, the completion result would always be COMPLETED, and only the "goodness" result would be determined by the results of the branches. Using two values creates other problems though, and not all combinations make sense. So all that to say, I agree that the behavior here with NOT_BUILT feels wrong, but I'm not sure if it makes sense to try to change it.
Based on this response, I don't think there's any way to fix this bug from within the Text Finder plugin or even the Matrix Project plugin. The flaw is fundamental to the semantics of the Jenkins Result API.