So, was browsing through the DivestOS FAQ and saw this little bit. Kind of fits the situation maybe?
Why haven’t you completely changed the default DNS servers?
Currently Quad9 (after trying many other public resolvers) is used for DNS fallback and for tethering. Switching it by default for cell would break VoLTE, SMS, MMS, and Visual Voicemail. And switching it for Wi-Fi could potentially break access to some LAN devices.