In browser developer tools it looks like the issue is coming from a custom css rule for dark theme that adds background-color=inherit to the seat diagram template output.

More specifically by disabling this rule local environment works as expected:
https://github.com/wikimedia/mediawiki-services-mobileapps/blob/master/pagelib/src/transform/ThemeTransform.less#L67

I am not sure though if its something that need to be fixed in the template level to allow dark theme to render properly or in the pagelib css level.

There are a bunch of div elements that should have notheme class to avoid this issue on dark theme: https://de.wikipedia.org/api/rest_v1/page/mobile-html/Vorlage:Sitzverteilung?theme=dark. I'd rather apply a specific class to the template element itself and then recursively add notheme in mobile html then.

Upd: the reason that the first diagram in the example above is not broken on a dark theme, is probably because div tags don't have a text node inside. This might be helpful in terms of solving the issue.