Yes, because testwiki isn't on Parsoid default, the legacy 2010 wikitext editor won't use Parsoid and so the Parsoid-only function call won't trigger. I don't consider this a bug.

(This definitely isn't "our" fault, but Parsoid, but I'm pretty sure this is working-as-designed.)

The same thing also happens in dag.wikipedia.org, which, to the best of my understanding, is Parsoid-default, but I might be wrong:

Preview of wikifunction from the source editor is not yet possible. Checked on testwiki and dagwiki with VE 2022.

I think this is a fundamental problem with the legacy 2010 editor never asking Parsoid to render previews, regardless of the wiki's or user's settings. To investigate.