Ah ha! @dmertz, you're a wizard! I went in and double-checked, and the 1099-R has a code T in box 7 and box 2a is blank - and so I had left the box 2a in Turbotax blank as well, which is how it imported. However, when I edited the form to insert a ZERO in box 2a instead of leaving it blank, the tax recalculated itself properly! I still think this is kind of buggy and the good Turbotax folks should figure out how to fix it so it doesn't calculate tax on a blank 2a, but at least my problem is solved 🙂
