Symptom:

At the English Wikivoyage, content that looks like this:

{{pagebanner}}
First line

is presented in the visual editor as being part of a complex transclusion. See https://en.wikivoyage.org/wiki/Catalina_Island?veaction=edit for an example. Adding a blank line between the two lines causes the two elements to be treated separately (i.e., as expected by a user). See https://www.mediawiki.org/wiki/Topic:Uvtkb6d0li09t3as

Bonus problem:
The page banner isn't shown in https://en.wikivoyage.org/api/rest_v1/page/html/Catalina_Island