Can you describe better what you mean by "this appears to be different from other wikis"? The following two API requests return the same structure for me:
https://en.wikipedia.org/w/api.php?action=parse&format=json&page=OLP...
https://sv.wikipedia.org/w/api.php?action=parse&format=json&page=Apa...
Note that there is also a `&parser=` option to both of these queries. `parser=parsoid` currently generates different output than `parser=legacy` for these queries, which should be fixed by https://gerrit.wikimedia.org/r/c/mediawiki/core/+/1264213 which is riding the train this week.
The output for article titles changed in 2022 (T306440) and then again in November of last year (T36514) with the title on the "edit" page being fixed more recently (gerrit 1239559).
But in this case the issue flagged seems to be the fact that the title code in Article doesn't match the code which is run in ApiParse for the displaytitle. Article writes to the OutputPage, while ApiParse displays the value in the ParserOutput. Only the OutputPage code path currently passes in the title language, which could probably be fixed.
Another issue is that {{DISPLAYTITLE:xxxx}} and/or -{T|....}- don't actually separate the namespace from the title, and so the output is very different when you use either of those functions in wikitext. That's T314399 (OutputPage::getUnprefixedDisplayTitle() is unreliable) which is its own fun time.
--scott