Thanks for your feedback. In Diamond Hunt the 50 points is awarded for crossing the middle screen only if you are playing just Game B. If you're playing all three games together (Game ABC) then it doesn't add 50 points.
Thanks for reporting your issue with Donkey Kong Jr. I wasn't able to replicate the problem in a few quick playthroughs that I did just now (I was able to unlock all 4 chains without any problem), but I'll keep trying to see if I can recreate your scenario with more testing.