In this example, the user has an instrument track “High strings” and a MIDI track “Low strings” routed to “High strings”.
When the user disables the instrument track, the routed MIDI track loses knowledge of its target.
When the user re-enables the instrument track, the MIDI track will still not have access to its target however, and seems to store only a name (“Missing:…”), rather than actual reference to its target.
This is needlessly frustrating and arcane behavior, and should be fixed.