Also related to this new/old divide, any problem-answer-variant combination that is last submitted before early March 2014 and has a variant (i.e. uses the Randomize Studio option) will have an empty value for "Question".
If the problem did not have variants, then the value of "Question" from the most recent problem part is copied into all entries for that problem part.  But since the value of the question may be different across variants, it is not copied in this way.