@Kusma pointed me to a thread on the English Wikipedia's technical village pump regarding a page (User:Bearian/Deletions, admin-only link) which appears to have taken 3 years to appear in a category (a deletion requestion category), initially thinking they couldn't see a reason for it due to an suppressed revision etc.
There was no suppressed revision, but checking logstash for the page title I found 5e325f4f-324a-406d-9823-1e560a619dfd, which shows Bot1058 requesting a RecursiveLinkPurge for the page, probably related to this edit going by the timestamps.
The time between the purge (Mar 28, 2022 @ 10:01), and the deletion (Mar 28, 2022 @ 11:54), suggest that this purge caused the page to appear in the category.
For some reason, a page tagged with a template (db-user) which should have added it to a category (Category:Candidates for speedy deletion) in July 2018 (diff, admin-only link) did not have its cache purged at edit, nor since, until today.