Under what circumstances would you recommend to change or not to change the preset DoH servers in the MB? You emphasize the use of MB with any VPN provider, but you also suggest that if a VPN is used, no other DNS servers should be used, even if encrypted.
Using MB with a VPN creates a contradictory situation as at the same time the preset encrypted DoH servers in the browser are active.
What should users do in this scenario?
You should use the browser with its default settings.
i am also confused as to why a custom dns was set by default in MB , be it a DoH one.
I believe while you are using a VPN with MB its best to just use the dns server provided by that vpn server by default. (whether or not DoH)
If a custom dns server is set , it may chose different dns server for different users based on their location.
This may have an adverse effect on the crowd blending approach for MB and would instead make them standout from others based on the dns server selected.
If anybody has any explanation as to the decision to keep this default in MB , i would like to know.
I mean, it’s common sense that you shouldn’t be changing this. Unless this is a mistake and they messed up, then you need to report this to them.
For the record , mullvads DoH servers are not the same DNS servers used by its VPN servers by default.
Keeping any fingerprinting concerns aside , when you are trusting a VPN provider , there is little reason to use DoH as you trusting the VPN company to not involve in something shady like dns spoofing .(which what DoH is essentially made for)
That being said using DoH of the same VPN provider won’t provide you any benefits over using plaintext dns of the same VPN provider as you’ll be trusting the same entity for not manipulating your DNS requests be it DoH or plaintext.
This differentiates us from the dozens of users who use the VPN provider’s DNS server.
I think an explanatory statement should be added to the recommendation page about which is the most reasonable way to use MB.
The important thing is to stay the same as the other Mullvad Browser users, it’s less about the other users on your VPN.
When we say…
Therefore, it is imperative that you do not modify the browser at all outside adjusting the default security levels. Other modifications would make your fingerprint unique, defeating the purpose of using this browser. If you want to configure your browser more heavily and fingerprinting is not a concern for you, we recommend Firefox instead.
…we mean it! 
But I can definitely update the site to include a mention about Mullvad Browser’s default DNS configuration as well, so people aren’t confused when they see this. 
Mullvads extenstion on MB itself prompts to disable DoH when on mullvad VPN .
Maybe you can clarify with them to clear the confusion 
