There was a related issue beforehand – but this isn't getting copied, so I can't work out out data-ve-ignore is getting added to the HTML?

This extra HTML is correctly added here for the sake of putting something meaningful into the external clipboard. data-ve-ignore tells the converter to ignore it if it's pasted into another VE instances, and about="g[random]" tells Parsoid to ignore it. However the about grouping appears not to be working at Parsoids end:

HTML:
<p><span about="g1" typeof="mw:Transclusion" data-mw="{&quot;parts&quot;:[{&quot;template&quot;:{&quot;target&quot;:{&quot;href&quot;:&quot;Template:Interprogetto&quot;,&quot;wt&quot;:&quot;Interprogetto&quot;},&quot;params&quot;:{&quot;commons&quot;:{&quot;wt&quot;:&quot;cats&quot;}}}}]}" data-ve-no-generated-contents="true"></span><span data-ve-ignore="true" about="g1"></span><ul data-ve-ignore="true" about="g1">
<li>Foo</li>
</ul>
<span data-ve-ignore="true" about="g1">

</span></p>

Wikitext:
{{Interprogetto|commons = cats}}

• Foo

<span data-ve-ignore="true" about="g1"></span>

The first about-grouped sibling is discarded, but the subsequent ul and span are not.

So this is caused by new templates always being inline templates, as we don't know if they are block templates until they render. As a result the above HTML is invalid (<ul> inside a <p>) which after Parsoid fixes, means the about-grouped items are no longer siblings.

This is scenario #3 from https://www.mediawiki.org/wiki/Parsoid/DOM_notes#Efficient_re-rendering_on_edits

Is Bug 60375 related?

Moving it back to VE.

(In reply to comment #5)

Is Bug 60375 related?

Looks like it.

Change 111518 had a related patch set uploaded by Esanders:
Only generate transclusion contents for clipboard

https://gerrit.wikimedia.org/r/111518

Change 111518 merged by jenkins-bot:
Only generate transclusion contents for clipboard

https://gerrit.wikimedia.org/r/111518