This normally indicates a router problem. Depending on the model of router, there are a number of ways the router can provide an alternate DNS path other than the Pi-Hole.
Look through all the menus on the router and look for other DNS settings (WAN, LAN, family-filtering, DNS pre-binding, DNS caching, dynamic DNS, IPV6). You may have to solve this through trial and error. Sometimes a setting is buried in a gray-out menu, and when you select the menu and make it active you find an underlying option that is causing the problems.
A second potential problem is DNS caching on clients. This does not appear to be the case on the Mac, since when it goes directly to Pi-Hole for DNS it works. But, Windows PC's can hang onto DNS settings unless you flush the DNS cache.