https://it.wikipedia.org/w/index.php?title=Se&action=edit has an intro section line: '''Se''', '''Sè''' o '''Sé''' possono riferirsi a:
https://it.wikipedia.org/w/index.php?title=Capricci&action=edit does not have such an intro section, it starts directly with a heading.
I'm afraid this is the reason. Whether it makes sense is a different question. :)

See the extracts in https://it.wikipedia.org/api/rest_v1/page/summary/Capricci versus https://it.wikipedia.org/api/rest_v1/page/summary/Se

Page previews uses the REST API, which uses Extension:Disambiguator. So something is going wrong somewhere in that workflow (likely the latter extension). Are you using the DISAMBIG magic word?

Looking more closely the API does seem to be reporting type "disambiguation" so perhaps there is an error in page previews after all. Not sure what's failing here.

Yes, the __DISAMBIG__ magic word is provided via the Template:Disambigua, used in both pages.
Could it be related to this? It looks like if the extract is empty (i.e., no lead section) it always returns TYPE_GENERIC.

Yes that looks l like that's what's happening,

https://it.wikipedia.org/api/rest_v1/page/summary/Capricci is returning an empty string for extract.
https://github.com/wikimedia/mediawiki-extensions-Popups/blob/master/src/preview/model.js#L155 returns undefined since its an empty string.

I guess that logic is probably incorrect.
As a workaround introducing a paragraph to the top of https://it.wikipedia.org/wiki/Capricci page should rectify the problem.