If the iCloud Private Relay ODoH DNS server is used it will show up as a DNS leak, even if the IP address from its response isn’t used for browsing. For privacy it doesn’t matter, as with ODoH the DNS resolver doesn’t know your IP or identity, the most important thing is whether it will bypass the NextDNS blocklist. In my testing I couldn’t visit any website that was blocked by NextDNS, meaning that the iCloud DNS resolver wasn’t used as the primary DNS resolver, which matches with their documentation (that page 10 that I linked to earlier). Note that Apple will only use a custom DNS resolver if you’re using the native DoH option, so for example the configuration that you can get from apple.nextdns.io.
You can easily test it yourself: block a hostname in NextDNS that you haven’t visited recently (due to cache) and try to visit it in Safari.
I don’t know why Apple still uses the Cloudflare DNS resolver even if it seems to be ignoring its responses. Maybe they use it for some custom metadata that’s sent along with the request which somehow is important for the relay. All I know is that I’ve never seen it bypassing the NextDNS blocklist, which again is exactly how it’s documented by Apple.