The issue is that according to the spec the two DNS servers provided by DHCP are equivalent. While most clients favor the first one as the default, that's not universally the case and when and how it switches to the secondary can vary by client (and effectively appear random). So you won't be able to know for sure which client uses your DNS, especially after your DNS server was unreachable for a while for whatever reason. Personally I've "just" gotten a second Pi to run redundant copies of PiHole, but only having a single DNS server is usually fine as well.