I've noticed this on test2.wikipedia.org where the extension was enabled.
The subtitle is meant to be inside a div#contentSub element, which is styled with a grey text color and a smaller font-size.
However, on wikis where WikidataPageBanner is enabled the subtitle is absent, and its contents are instead output inside a custom div.ext-wpb-pagebanner-subtitle element, which is missing the correct positioning and styling.
Traced it down to the following code:
- rEWDP7d39c374cc58: Allow skins to render subtitle after banner
- T113642: Should be possible to print breadcrumbs above the heading to avoid reflows on Wikivoyage