In macOS 15.4, with the latest ARK, it was necessary to turn off System permissions to access the microphone for (Audio Hijack and) Audio Routing Kit (ARK), allow quit & relaunch, then turn on permission again, allow quit & relaunch, in order for the Mac to see virtual audio devices in Loopback. It didn't matter that Permissions sections of apps had permissions installed.
Symptom is that Loopback devices appear missing when running Audio Hijack tasks.