Ah! Tarzana was heavily modified - in a way that deleted its original relation record: Relation: 11002094 | OpenStreetMap
Further searching shows that whoever did that edit fully removed the city, so some Large OSM Edits™ are needed to bring that back. Until that’s done, CityStrides will get no updates to changes in Tarzana.
I was able to see this by opening the three-dot vertical menu on the city page, click on the link for OSM, and review the details on that page (its latest update). It’s still a little obfuscated, because you have to notice that the map on that page doesn’t include a city border.
[edit] oh wow, I think that edit removed all nested cities from OSM