This site diversity change means that you usually won't see more than two listings from the same site in our top results. However, we may still show more than two in cases where our systems determine it's especially relevant to do so for a particular search.... — Google SearchLiaison (@searchliaison) June 6, 2019

Since the new feature is domain-based, it will treat subdomains and their root domains as part of a single website. Google will still only show two top results from them and will only treat them as separate websites if it finds more than a couple of highly relevant pages. As people pointed out on Twitter, it doesn't always work yet -- Google still returns a bit too many listings from a single domain at times. Danny Sullivan, Google's public Search liaison, said the company will keep working to improve it over time.