The Scribunto Lua reference manual states that /doc subpages in the Module namespace have their page content models set to wikitext and not Scribunto, so they do not accept Lua code by default, and the standard wikitext editor loads rather than the code editor.
For some reason, I believe after the MediaWiki 1.33 upgrade, whenever I attempt to create a /doc subpage in the Module namespace, the content model is set to Scribunto and the code editor loads. Normal expected behaviour is that the subpage would be in wikitext and the standard wikitext editor loads when creating the page. Now, I need to create the /doc subpage empty and change the content model back to wikitext before I can continue creating the /doc page.
This appears to be affecting all wikis. An example here: https://meta.miraheze.org/wiki/Module:Foo/doc?action=edit (you'll notice that the code editor rather than the wikitext editor loads).