Some Fandom Wiki puts a quote(it has empty <p> tags) at the beginning of a page, I guess this causes the MediaWiKi API returns an empty string. The API I'm using is this:
https://www.mediawiki.org/wiki/Exten...xtExtracts#API . If you see the first section of a page is not a normal paragraph like Wikipedia, it's very likely the API will return nothing.
You could ask the admins of these fandom wikis for help, a normal user may not have the permission to solve the problem.