Subiquity basically uses an old snapcraft.yaml configuration. I updated this configuration to use base: core18 and multipass and set up all of its dependencies and packages to be local. When injecting the snap into an ISO, snapd gets stuck in "Wait until snapd is fully seeded". Checking back with TTY2, I see that the /snap/bin directory is empty. When not specifying a base keyword, the /snap/bin directory consists of the required snap parts and runs correctly.