I found this very interesting and did some research on the Internet. Apparently there are some other death masks of Luther. The most well-known one is on display at Halle in Germany. Pictures of this mask can be seen online and depict a man with very strong features, much more believable as Luther, and not at all matching the death mask we saw at the seminary.
I haven't contacted the seminary library with my questions about this but either they have the wrong death mask or they're keeping the real one in a safer place.
Since the "death mask" is made by taking a cast of the deceased person's face, this is more likely to be the real deal, not a sculpture showing a face strong and vibrant.
Some death masks? How many? Did it occur to you that it takes awhile for plaster to dry? One mask could have been made the day the man died and the last could have been made days later. The first one could have been of a healthy looking person and the last could have looked like a guy who has been dead for three days.
Have you researched "life mask"? Lincoln had three of those, IIRC.
yitbos