If I don't set clshow=!hidden, then a continue element is returned as expected. The same example query, with no clshow parameter.

It seems likely based on the effect and timing that the issue is associated with this change: https://gerrit.wikimedia.org/r/c/mediawiki/core/+/1241311 (@Zabe) which changed how categories are filtered.

The impact is:

• Action API requests with clshow=!hidden or clshow=hidden that correctly returned a continue element until ~27 February 2026 now fail to do so (i.e. this is a regression)
• the API silently fails to return all categories for a valid request
• as there is no continue element, the user has no way of knowing that they have not received all requested data, nor any way of retrieving said data without altering their requests

The bug here is that in the old query design the filtering happened while querying, such that after the filtering we had $params['limit'] + 1 results. Now we query $params['limit'] + 1 items and only then do the filtering, such that we can end up with less items (which also causes the continue element from vanishing).