According to cache partitioning[1] practices, this is correct because the cache key has changed, so each iframe embedded on different webpages will be cached separately. However, the cache size for YouTube's iframe is excessively large. I'm unsure exactly what it's caching, but it absolutely cannot consist solely of static resources.
[1]: https://developer.chrome.com/blog/http-cache-partitioning#how_will_cache_partitioning_affect_chromes_http_cache
