Category membership is only updated on link updates (edits, null edits, template edits, purging with api with fotcelinkupdates option). You cannot use template trickery to change the actuak categories unless you are ok with much delayed updates.

Closing wontfix as I don't think this is likely to change

Thank you Brian, but why can't this be fixed?

Why can't a page have its category membership updated upon a purge, in case it is detected that the categories it belongs to have changed upon purging?

I don't see why this should be "unlikely to change".

[Ni!]'s

q.

I suppose some of this is just because its the way things have always been done. Im not 100% sure of the exact reasoning for this.

Part of it is probably that pages get rerendered quite frequently and in an unpredictable manner. If categorylinks were updated on every page parse it would be possible to construct a page that changed its category membership on every (logged in) page view which would probably be bad. Additionally the vast majority of pages do not have their categories change unless their page text has changed.

Terminology note: having categories update from purge requests (and not general page render as suggesyed in comment 0) is a bit more likely to happen (although still not something I would probably commit)