Well that sort of led me down a path.
I have a Synology 2600 router with a Spectrum-provided Cisco DPC3216 cable modem. There is no way to set DNS or anything else on the modem. The router has sections labelled "Internet" and "Local Network".
I had nothing specifed under Internet/Manually configure DNS server. I had my piHole IP address (only) under Local Network ipV4, but DHCP is disabled as I'm letting piHole handle DHCP.
For Local Network ipV6, I have it enabled to pull in the prefix, but it is set to Stateless mode with a specifed DNS pointing to the fe00:: autoconfigured address for my Pi.
--EDIT-- Actually the setting I use is "Stateless DHCPv6 mode" which according to Synology automatically configures ipV6 addresses but pulls DNS from DHCPv6, which I specified as my Pi's address.
As a test, I went to "Internet/Manually configure DNS server" and changed it from nothing to the local IP address of my Pi. This caused subsequent attempts at loading pi.hole/admin to query checkip.synology.com and checkip6.synology.com, then eventually fail with a DNS_PROBE_FINISHED_NO_INTERNET in Google Chrome web browser.
So it appears if an address cannot be resolved, Synology eventually sends the query to whatever it's getting from my ISP as the default DNS -or- what is specifed in Manual DNS on the router.
The true root of the problem, though, is why can't I resolve http://pi.hole/admin on Apple devices when they have the same (apparent) DNS setup as Windows machines that work?