if you look at how MidiOutput/input::GetDevices() works, it makes a system call. It seems that the problem is with apple’s code not updating the list of devices. Have you tried looking at Audio Midi Setup’s MIDI window when you create your device via that code to see if it shows up there? Another option is checking with Midi Monitor (https://www.snoize.com/MIDIMonitor/) to see if your MidiDevice shows up there when your app is executed.
probably should let @jules @timur know about this odd behavior.