Wikibase gate-and-submit builds are failing (example build) because the action=parse API returns an unexpected HTML comment now:
AssertionError: expected '\n<!-- Saved in parser cache with key wikidb:pcache:idhash:29-0!canonical and timestamp 20211109100948 and revision id 34. Serialized with JSON.\n -->\n' to equal '' + expected - actual - -<!-- Saved in parser cache with key wikidb:pcache:idhash:29-0!canonical and timestamp 20211109100948 and revision id 34. Serialized with JSON. - -->
Usually, this comment is only there when the disablelimitreport parameter is not set. Compare this English Wikipedia request with this Beta English Wikipedia Request using the same parameters – the Beta version has this extra comment as well:
\n<!-- Saved in parser cache with key enwiki:pcache:idhash:1-0!canonical and timestamp 20211109093033 and revision id 512115. Serialized with JSON.\n -->
Note that this only happens when using page=, not with text= + contentmodel=wikitext. (Which makes sense – with text=, you wouldn’t expect the result to be stored in the parser cache.)
I think this is a regression in MediaWiki core that should be fixed there, rather than being handled in the Wikibase API integration tests – other API users might also expect the action=parse&disablelimitreport= result to not have this comment. Move limit report rendering to ParserOutput looks like the most likely culprit to me.