Not a bug (yet 
), but a wiring error. I’m not sure how fritzing is detecting it, but the ground wire on the bottom of the breadboard with the amphenol connectors is missing. Adding that completes routing. Found by using the delete minus function to delete parts one by one until routing completed. That happens when that particular breadboard is deleted. Then looking at only that wiring indicated the problem. There are however more problems that I am still looking at. The opto isolator circuit is incorrect (it won’t work as it stands). It looks to be backwards the input should be coming from Din2 pin 4 in to the diode on the opto isolator and the output from the opto isolator (currently going to Din2 but with no ground) should go to the rx data pin on the mega. This assumes that pin 4 of Din2 is TX data from whatever connects to Din2 (as seems likely). As currently connected this won’t do anything. In the updated sketch below there are still some routing problems in schematic in that di for the neopixel is connected to +5V although it is not in breadboard. It is possible the neo pixel part is in fact incorrect, I’ll have a look at that next.
Chewie 2_fixed.fzz (90.5 KB)
Peter