This is consistent between Parsoid and PHP:

$ echo '[[[Main Page]]]' | bin/parse.js --normalize
<p>[[[Main Page]]]</p>

-vs-

$ echo '[[[Main Page]]]' | php maintenance/parse.php 
<p>[[[Main Page]]]
</p>

So that's pretty much "how it's supposed to work". Changing this would be wikitext 2.0-scope work.

In T220055#5084977, @cscott wrote:

So that's pretty much "how it's supposed to work". Changing this would be wikitext 2.0-scope work.

Thanks for that info! Would a good first approach be fixing the HTML to WikiText parsing?

$ echo '[<a href="/wiki/Main_Page" title="Main Page">Main Page</a>]' | node parse.js --html2wt
[[[Main Page]]]

It should instead output <nowiki>[</nowiki>[[Main Page]]<nowiki>]</nowiki>