Amazon kindly sent me a new copy despite being outside the 30 day window, however, the issue remains on this set so I'll send this back and keep my original copy.
It's odd, it only appears for about 4-5 seconds, but I thought it was a problem with the disc. It may just be on the film print, or it's a layer change issue, or part of a bad batch, but I have no idea what it is. Or perhaps it's only on the blu ray and not the 4k.
Out of interest, would somebody mind looking out for this on your copy of The Godfather Part 2 when watching this on either 4k or regular blu ray? Couple of photos below. Happens at 1 hour 46 minutes 27-32 seconds (roughly).