I can confirm this issue does occur. I don't think it is a bug in the scaffolding code though. It is a bug in line 506 of ContentGenerator/Feedback.pm. That code should check for undefined keys. The database stores all of the answer blanks and null values if the part has not been answered. This is not a new bug to 2.15. I also tested this with 2.14, and the same issue occurs.
I believe that this is caused by the answer blanks being hidden by the scaffold. As such their values are not submitted with the problem form. I have another type of problem that disables inputs in another way, and the same thing happens with those.