I can't reproduce this issue.

In MediaWiki core, on e.g. User:Root/foo.json (with https://gerrit.wikimedia.org/r/177172 applied), this works fine:

In EventLogging, on e.g. Schema:Edit, this works fine, too:

Ah, so those were pre-existing pages that I edited to invalidate any caches, but the page itself already existed. And that includes the content-model property of a page.

When creating a new page on latest MediaWiki, it is indeed broken. Verified it is caused by a90b4c58099. Works after reverting that. Reverted in master, and recommending backport in next SWAT.

As @Nuria noticed, it only affecting new pages was the case for simple MediaWiki installs. But in production it did actually affect existing pages, too. This because $wgContentHandlerUseDB (which makes the content-model bound to a page) is disabled in WMF's mediawiki-config.

Change 185096 had a related patch set uploaded (by Legoktm):
Fix display of Schema pages

https://gerrit.wikimedia.org/r/185096

Patch-For-Review

• JsonContent was changed to return objects instead of assoc arrays, it also (unintentionally) broke the ability of extensions to extend JsonContent ("looked" fine in EL when creating a new page with $wgContentHandlerUseDB = true, MWException if it was false)
  • What was really happening was that new pages were being created with the JsonContent model instead of JsonSchema.
• a90b4c58099 fixed the ability for extensions to extend JsonContent. This meant that pages would now be created as JsonSchema.
• JsonSchema had a broken display due to changes of array -> object. It appeared that reverting a90b4c58099 fixed that, except it was really just creating pages as JsonContent.
• My patch fixes an is_array -> is_object, which should fix the display.

See also T86270: Add support in JsonContent for non-object structures (e.g. array) caused by the same patch.

Change 185096 merged by jenkins-bot:
JsonSchemaContent: Fix html rendering of objects and arrays

https://gerrit.wikimedia.org/r/185096

Change 185363 had a related patch set uploaded (by Ori.livneh):
JsonSchemaContent: Fix html rendering of objects and arrays

https://gerrit.wikimedia.org/r/185363

Patch-For-Review

Change 185364 had a related patch set uploaded (by Ori.livneh):
JsonSchemaContent: Fix html rendering of objects and arrays

https://gerrit.wikimedia.org/r/185364

Patch-For-Review

Change 185363 merged by Ori.livneh:
JsonSchemaContent: Fix html rendering of objects and arrays

https://gerrit.wikimedia.org/r/185363

Change 185364 merged by Ori.livneh:
JsonSchemaContent: Fix html rendering of objects and arrays

https://gerrit.wikimedia.org/r/185364

Thanks Timo.