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.

Fixed on 7.8.0 (5589)

Tested on iPhone on iOS 26.0 and iPad on iPadOS 18.5. Tested on multiple articles.