So I found another post similar to mine AX1800, Mullvad DNS, unable to resolve some domains and it was confirmed that indeed the DNS server 193.138.219.228 has been hard-coded.
Please be advised that DNS address is no longer in use since 2019 as per this post Our public DNS is changing | Mullvad VPN
Long story short, I have to create my own group for Mullvad to customise the servers config, so what's the point of providing a list of pre-installed VPN providers if you don't keep their configurations up-to-date?
This undermines the benefit of offering pre-configured VPN options and adds unnecessary complexity for users who expect these integrations to work out of the box.
I strongly suggest that you regularly update the configurations for all listed VPN providers, or at the very least, provide a warning or clear documentation when certain setups require manual intervention.