Agreed, though I don’t think they disabled systemd-resolved, because it still works using 127.0.0.53 when they’re connected to the VPN. So the daemon must be running, unless Mullvad itself has a DNS forwarder using the same loopback. I suspect they either hard coded some upstream DNS server for Mullvad, because Mullvad might not have supported systemd-resolved yet. Or maybe they set a permission on the configs, and something changed with the user context of Mullvad processes.

Interesting that it works when the VPN is connected, though. I also believe that systemd-resolved is installed on just about any system using systemd, but often isn’t enabled, without problem. Enabling it would generally involve a resolv.conf symlink and a config, so maybe that config was hijacked by Mullvad (or OP configuring Mullvad), and there’s no upstream DNS server available when the VPN isn’t connected.

I missed that it’s Linux Mint in the original post, and it looks like Mint has started using systemd-resolved. The Arch wiki might be useful to OP on how things are configured:

https://wiki.archlinux.org/title/Systemd-resolved

Ok, so something setup 127.0.0.53 as your DNS server, and isn’t removing it correctly. I think it’s safe to say it’s Mullvad, since it works using that DNS server IP when connected. Is that IP in your resolv.conf, or is resolv.conf maybe a stub, and you’re using systemd-resolved?

Ok, so does the VPN bring it’s own DNS? Some VPNs do, so it may explain why everything suddenly works fine when you connect.

When not connected to VPN, are you able to dig or nslookup internet names? Local names? A server timeout will be very different from an nxdomain or an empty SOA, in the response.

Are you able to telnet to a public web server on TCP/443?

One thought I’m having is, maybe at some point you set a static IP on your wifi interface, but screwed up the subnetting.

Have you ever messed with network manager or systemd-resolved internal settings, maybe trying to setup multicast DNS or caching?