While we wouldn't want to make any assumptions about the vehicle's authenticity, that is frequently the cause of lots being pulled or vehicles failing to attract interest from bidders. Hemmings relates the words of Philip Powell at Classical Drive who supports the notion that the postponement could be a sign that the Type D is not 100% kosher.
The car's history is known, from being totally rebuilt after being reduced to just a driveline in an accident, to being found in the former Soviet Union, but the questions can linger in the minds of potential owners. It should be noted, however, that Christies is investigating the car's race history, not its authenticity, per se. We saw something similar in Arizona last month when a rare Ferrari Dino SP failed to crack the $1 million mark and didn't meet its reserve, going unsold. The car had been well-documented, but because it had been nearly totaled in a racing incident, the rebuilt car can sometimes raise doubts among buyers who would rather have the genuine article than one rebuilt by a non-factory team years after its birth.