After closing the window and leaving the app in the background, the next time you open it the connection to the MIDI ports is lost. #2041
Description
rec2.mp4
As can be seen from the attached video.
Refreshing the MIDI ports from the preferences restores the connection.
A solution, of course, would be to programmatically refresh the ports every time you start playing (although this might be excessive, since the problem only occurs when closing and reopening the app).