When opted into beta and viewing the lead paragraph...
https://en.m.wikipedia.org/wiki/Planet is a rather interesting specimen. The lead paragraph is the text "A planet is an astronomical object orbiting a star or stellar remnant that". It is followed by a list item.
In the apps (see T111958 for historic purposes) they work around this issue by combining any trailing elements after the first paragraph (but there is also some logic to check paragraph length)
~~I don't think this is the right behaviour.
Using the 2nd paragraph and 3rd child element (after `p`,`ul`), changes the order and flow of the content that our editors have agreed upon.~~~ [edit this doesn't relate to this particular problem my bad!]
Possible alternative solutions:
 Do nothing and accept the edge case.
 Move any elements between the lead paragraph and the next paragraph element that are not themselves paragraphs.
I prefer the latter.
[[ https://github.com/wikimedia/mediawiki-services-mobileapps/blob/master/lib/transformations/extractLeadIntroduction.js | Sample code for Mobile content service ]]