Interestingly, the supplied example.zip was testing org.junit.platform.commons.util.AnnotationUtils.findRepeatableAnnotations from JUnit 5 instead of org.springframework.core.annotation.AnnotationUtils.getRepeatableAnnotations from Spring. In any case, the error is the same. Neither Spring nor JUnit 5 find the repeatable annotation for the "multi" scenario.
Actually, after having put further thought into it, the expectation for AnnotationUtils.getRepeatableAnnotations (as well as for JUnit's similar method) is invalid.
getRepeatableAnnotations does not merge annotation attributes. Thus, the algorithm may encounter @A twice, but each encounter is seen as the same instance. The @A annotation is therefore only found once. Consequently, the expectation should be 1 instead of 2 for the non-merging search algorithms.