You see the crumbtrail while editing, but after editing it's gone until you reload.
The DOM has a <div id="contentSub"> but it's empty. Upon reload you see the expected
<div id="contentSub"><span class="subpages">&lt; page links </span></div>

When you save the API calls $this->getOutput()->getSubtitle() to get the new text for that, but it's returning nothing... I guess we used to trigger something that would set it.

Friendly bump. This is a minor issue but still slightly annoying :)

Are you absolutely sure this is a regression? Looking at the core code, I don't see how this ever worked.

I figured out how to get it working with 10 lines of hacking around MediaWiki's context internals though

In T97700#2171063, @AlexMonk-WMF wrote:

Are you absolutely sure this is a regression? Looking at the core code, I don't see how this ever worked.

No. :-)

Change 281463 had a related patch set uploaded (by Alex Monk):
Horrible context hacks to make subpage breadcrumbs show after save

https://gerrit.wikimedia.org/r/281463

Change 285428 had a related patch set uploaded (by Alex Monk):
Allow specifying OutputPage object in Skin::subPageSubtitle parameters

https://gerrit.wikimedia.org/r/285428

Change 285428 merged by jenkins-bot:
Allow specifying OutputPage object in Skin::subPageSubtitle parameters

https://gerrit.wikimedia.org/r/285428

Change 281463 merged by jenkins-bot:
Fix display of subpage breadcrumbs show after save

https://gerrit.wikimedia.org/r/281463