Using windows 10 1909, IPv4 and IPv6 both enabled.
environment: I'm using pihole + unbound on a raspberry pi as a local DNS resolver and have thus configured (DHCP) to use an IPv4 address as my DNS server. This works fine, goal: kill trackers and ads, privacy.

I discovered that, whenever my local DNS server becomes unavailable (powered down, crashed, maintenance, ), Windows 10 apparently triggers a process to search for external DNS servers, and unfortunately finds them (IPv6). If pihole crashed, without me noticing this, windows 10 continues to operate, using these external DNS servers, of course, all trackers and ads are operational again, I usually get alerted if I see to many ads, by then the trackers have done the damage.

The question: Is there a way to prevent this, in my case, undesired windows 10 feature, adding unwanted DNS resolvers to the system, thus invading my privacy?