Please do NOT touch the referenced article before investigating the issue.
Reproduction steps: Get https://en.wikipedia.org/w/api.php?action=query&format=json&prop=info|extracts|pageimages|revisions&formatversion=2&redirects=true&exintro=true&exsentences=5&explaintext=true&piprop=thumbnail&pithumbsize=300&rvprop=timestamp&titles=Hydrogen_damage&smaxage=300&maxage=300&uselang=content
Expected result: Parameter extract contains a meaningful plaintext extract.
Actual result: Parameter extract is an empty string:
{"batchcomplete":true,"query":{"normalized":[{"from":"Hydrogen_damage","to":"Hydrogen damage"}],"pages":[{"pageid":3605505,"ns":0,"title":"Hydrogen damage","contentmodel":"wikitext","pagelanguage":"en","pagelanguagehtmlcode":"en","pagelanguagedir":"ltr","touched":"2016-02-12T01:36:19Z","lastrevid":695808952,"length":8508,"extract":"","revisions":[{"timestamp":"2015-12-18T19:21:17Z"}]}]}}Reproduction steps: Get https://en.wikipedia.org/w/api.php?action=query&format=json&prop=info|extracts|pageimages|revisions&formatversion=2&redirects=true&exintro=true&exsentences=5&piprop=thumbnail&pithumbsize=300&rvprop=timestamp&titles=Hydrogen_damage&smaxage=300&maxage=300&uselang=content (it is the previous URL with the explaintext parameter removed)
Expected & actual result: Parameter extract contains a meaningful HTML extract:
{"batchcomplete":true,"query":{"normalized":[{"from":"Hydrogen_damage","to":"Hydrogen damage"}],"pages":[{"pageid":3605505,"ns":0,"title":"Hydrogen damage","contentmodel":"wikitext","pagelanguage":"en","pagelanguagehtmlcode":"en","pagelanguagedir":"ltr","touched":"2016-02-12T01:36:19Z","lastrevid":695808952,"length":8508,"extract":"<p><b>Hydrogen damage</b> is the generic name given to a large number of metal degradation processes due to interaction with hydrogen.</p>","revisions":[{"timestamp":"2015-12-18T19:21:17Z"}]}]}}Reproduction steps: Get https://en.wikipedia.org/w/api.php?action=query&format=json&prop=info|extracts|pageimages|revisions&formatversion=2&redirects=true&exsentences=5&explaintext=true&piprop=thumbnail&pithumbsize=300&rvprop=timestamp&titles=Hydrogen_damage&smaxage=300&maxage=300&uselang=content (it is the first URL with the exintro parameter removed)
Expected result: Parameter extract contains a meaningful plaintext extract.
Actual result: Parameter extract equals \n== ClassificationsEdit == (observation: "Classifications" is the first section heading of the article):
{"batchcomplete":true,"query":{"normalized":[{"from":"Hydrogen_damage","to":"Hydrogen damage"}],"pages":[{"pageid":3605505,"ns":0,"title":"Hydrogen damage","contentmodel":"wikitext","pagelanguage":"en","pagelanguagehtmlcode":"en","pagelanguagedir":"ltr","touched":"2016-02-12T01:36:19Z","lastrevid":695808952,"length":8508,"extract":"\n== ClassificationsEdit ==","revisions":[{"timestamp":"2015-12-18T19:21:17Z"}]}]}}I have noticed this before in another article and a null edit (do NOT do that before investigation) fixed that. It seems that some secondary information (i.e. data generated automatically from primary information) is broken, but I would expect all secondary information to be regenerated periodically.