That is not quite accurate - you aren't running any DNS servers other than Pi-Hole. Something was likely pointing to another third party DNS server, bypassing the Pi-Hole.
There could be a number of reasons for a DNS path around the Pi-Hole. It could be your router, it could be the cache in the clients, it could be an IPV6 path, etc. That's why you work on one client at a time. I suspect that since your iPhone is able to connect to Pi-Hole, it's not the router (since the phone is going through the router), but client specific.
Read through some of the FAQs here, they have a lot of good information. One example: