Is compiling/testing/using a systemd-resolved not aligned with the version of the rest of systemd present on the system supported?

Solution 1:

No, only building and upgrading one binary of systemd is not supportable. As in, people you might ask for help will have difficulty reproducing what you are doing.

resolved, like the dozes of binaries that compose this thing, links to some systemd shared code. This is not likely to remain binary compatible across an arbitrary number of releases. It might work, but personally I'm not willing to untangle systemd's internal dependencies to prove it.

Instead, try upgrading all of systemd. For a start, try reproducing the problem on a distro with a relatively new version. In Red Hat land, as of 2021 this could be Fedora 35 or RHEL 9. Once there is evidence the newer distro improves things, then start isolating relevant changes. Or start an OS upgrade project.