VE is sending [[Link (pipe trick)|]] in the data-mw.params,

<span typeof="mw:Transclusion" data-mw="{&quot;parts&quot;:[{&quot;template&quot;:{&quot;target&quot;:{&quot;wt&quot;:&quot;1x&quot;,&quot;href&quot;:&quot;./Template:1x&quot;},&quot;params&quot;:{&quot;1&quot;:{&quot;wt&quot;:&quot;test [[Link (pipe trick)|]]&quot;}},&quot;i&quot;:0}}]}"></span>

Since that doesn't tokenize as a link,

> echo "[[Link (pipe trick)|]]" | php bin/parse.php
<p>[[Link (pipe trick)|]]</p>

Parsoid's escapeTplArgWT escapes the pipe as {{!}} in a template param context.

This probably falls under a duplicate of T139159 in deciding where PST is supposed to happen.

I feel like this has to be guided by TemplateData: we shouldn't be trying to do PST on all arbitrary parameter data coming from VE: PST is appropriate only if the particular argument is tagged as having type="content" or type="unbalanced-wikitext" etc. And once that type is present, it's arguable that VE should be presenting a rich editing widget and handling the link as HTML, in which case the pipe trick is moot. If the user *does* want a "source editing interface" for a wikitext-type template parameter, then I think arguably it should be the client's responsibility to do the PST on that content (if that's desired; see also T139159: Pre-save transforms aren't executed on VE<->WT switch, so e.g. signatures are nowiki'ed, pipe trick links are left unresolved) rather than necessarily pushing that into Parsoid.