Selecting a client via its MAC address is preferential because it is a unique selection. IP addresses can change (even often) depending on your DHCP server and you'd loose client recognition easily. Also, many users have frequently changing IPv6 addresses (may it be either due to the ISP sending out new prefix every day or by operating systems deeming a frequently changing address a "good idea" (Windows, MacOS)).
The fact that
may not have propagated correctly downwards into other regions of the code.
Thanks for the confirmation, the bug fix will be included in the next release of Pi-hole. You can just stay at this custom branch for now but make sure to switch back to master when / before upgrading to the next version of Pi-hole.