Full disclosure, I'm not a fan of systemd. I started working with Linux in the late 90's and watched it grow from a marginalized operating system to the most dominant operating system in the datacenter. I've lived through so many "year of the Linux desktop" years I remember when it wasn't a joke. From my vantage point, administering Linux servers professionally for nearly 20 years, systemd is Linux on the desktop at the cost of Linux in the datacenter.
Why do I feel this way? It's mostly the reinvention and incorrect implementations of core UNIX tools and modalities. There's a lot of information on systemd out there. There's a lot of bias involved. So, today, I'm not going to talk about that. I am going to address a critical mistake in the systemd-resolved daemon which implements DNS lookups for systems running systemd.
I'll jump right to the work-around. If you're running a system which is using systemd, you should probably be running systemd-resolved configured to use a single DNS resolver, 127.0.0.1, and run Unbound. There are resources on how to configure and run Unbound, but the best is Calomel's Unbound Tutorial. If you need to maintain consistent, reliable DNS resolution that's compatible with previous versions of Linux, the only way to do that is to have a single DNS server in /etc/resolv.conf.