Here's a nice test for you to try. Keep the arduino system powered up with its own supply.... usb right? Then keep the led strip and the strip's supply away from the arduinos. Eg. Put the isolated led strip and its supply on the other side of the room..... and turn it on. And see what happens with the arduinos on the other side of the room. If all ok.... then gradually bring the led strip closer and closer to the arduinos...... which will require getting the strip and its supply closer to the arduino. This also means that the arduino has no control or contact with the strip and the strip supply. If no issue.... then you can start focusing on wiring and/or component and/or noise and/or supply stability and/or decoupling issues.