I am having the same issue, and I have to assume it's one of my soldering joints and not a bad tube. The problem appears during the tube test in the instructions. The right most pair of digits are not displaying properly. I've gone back to fix a few solder connections at the base of the tube, but that hasn't fixed the problem. Thankfully it didn't make it worse, either. I also checked the connections at the right angle connector per the original poster's suggestion, but those all look perfect.
Is there a way to determine which connections correspond with the problem digits? If the defect is not visible, I really don't want to have to go back and resolder all of the connections, especially since I trimmed the excess wire already.