Can't see any extra interrupt processing here apart from the MIDI library. That is a rough tutorial, it is wrong. The capacitor should not be on the latch pin. This is an ongoing problem with these official tutorials. Many people have pointed out the problem but nothing gets done. Each chip should also have a 0.1Uf ceramic capacitor placed between the power pin and ground, as close to the chip as possible and with as short a lead length on the capacitor as possible.What does the (x24 meters) bit mean?
x24 means I took this shift out circuit and made 24 of them.