contentbylabel macro does not find all matching pages

We have several pages with a specific label. But contentbylabel doesn't list all of them for an unknown reason.

The metadata-plugin in fact works well. So it can be used as workaround:

Both should produce the same result but won't. metadata-report lists more pages.

It might be interesting to know, that the pages that should be listed are all siblings.

I also tried reindexing but it did not help.