UMH-10 MIDI USB Host

I have purchased the DigitalLife UMH-10 MIDI USB Host from Amazon. I have connected it to a Yamaha P-121 digital piano and it seems that the sending and receiving of MIDI works correctly. However, while the MIDI in light only lights up when signals are being received, the MIDI out light is constantly flashing even when the keyboard is not being played. Is this Normal? What could it be?

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Yamaha MX88