https://lists.wikimedia.org/hyperkitty/list/analytics@lists.wikimedia.org/thread/S3D72D4WYBFNGD3OHAV37MCN5MNESB5U/ says some pageviews data are missing for Oct 21, 2021. I quickly checked, and it looks they're right: https://wikimedia.org/api/rest_v1/metrics/pageviews/per-article/zh.wikipedia/all-access/all-agents/Cat/daily/2021102000/2021102200 does not offer any data for Oct 21, 2021.
Page "Cat" at zh.wikipedia was, however, visited six times on that day, according to multiple other data sources:
[urbanecm@stat1005 ~/tmp/zhwiki-pageviews-issue]$ grep '^zh.wikipedia ' pageviews-20211021-spider > pageviews-20211021-spider-zhwiki [urbanecm@stat1005 ~/tmp/zhwiki-pageviews-issue]$ grep '^zh.wikipedia' pageviews-20211021-automated > pageviews-20211021-automated-zhwiki [urbanecm@stat1005 ~/tmp/zhwiki-pageviews-issue]$ grep '^zh.wikipedia ' pageviews-20211021-user > pageviews-20211021-user-zhwiki [urbanecm@stat1005 ~/tmp/zhwiki-pageviews-issue]$ grep ' Cat ' pageviews-20211021-spider-zhwiki zh.wikipedia Cat 7535498 desktop 1 R1 [urbanecm@stat1005 ~/tmp/zhwiki-pageviews-issue]$ grep ' Cat ' pageviews-20211021-user-zhwiki zh.wikipedia Cat 7535498 desktop 2 A1G1 zh.wikipedia Cat 7535498 mobile-web 3 K1L2 [urbanecm@stat1005 ~/tmp/zhwiki-pageviews-issue]$ grep ' Cat ' pageviews-20211021-automated-zhwiki [urbanecm@stat1005 ~/tmp/zhwiki-pageviews-issue]$ # 2 user visits for desktop, 3 for mobile-web, 1 spider desktop visit => 6 visits in total [urbanecm@stat1005 ~/tmp/zhwiki-pageviews-issue]$ hive --database=wmf [...] hive (wmf)> select sum(view_count) from pageview_hourly where year=2021 and month=10 and day=21 and project='zh.wikipedia' and page_title='Cat' limit 1; Stage-Stage-1: Map: 68 Reduce: 1 Cumulative CPU: 2069.72 sec HDFS Read: 6321078164 HDFS Write: 101 SUCCESS Total MapReduce CPU Time Spent: 34 minutes 29 seconds 720 msec OK _c0 6 Time taken: 38.457 seconds, Fetched: 1 row(s) hive (wmf)> [urbanecm@stat1005 ~/tmp/zhwiki-pageviews-issue]$
Maybe the job loading data into the API (I think API uses Cassandra internally?) failed for some reason?