MIDI stopping after a couple seconds is a classic symptom of a MIDI feedback loop somewhere in your setup. You can usually fix this by tweaking the configuration in the app that is sending MIDI to Lumen.
If a program that sends out MIDI is also sending out whatever MIDI messages it receives, then it will send out more and more duplicate messages faster and faster after it sends the first one. macOS monitors the number of MIDI messages sent per second on the IAC bus, and if it gets too high, it shuts it that IAC bus channel down for a little while.
Because Lumen doesn’t send out any MIDI messages, the key is to look elsewhere in your setup. The best place to start is at the the program that is actually sending the MIDI data. Typically we see folks accidentally sending MIDI to the IAC bus on a channel that is also listening to all MIDI channels on the IAC bus.