When can anchoring backfire?
I think there is a significant confound in your experimental design: The anchored subjects only make 1 estimate whereas the "no anchor" subjects make 2 estimates. Note that on the first estimate the anchored subjects were much closer to the anchor than the non anchored ones, which is entirely consistent with Tversky's heuristic.

Also, "In all cases, Jelly Bean Counter 5000 provided the exact number of jelly beans in the jar." means that this isn't really a standard anchoring task. Normally, anchoring values aren't correct (in fact much of the anchoring work involves very incorrect anchors). This task confounds being close to the anchor with being close to the correct answer. 

You might have an interesting finding: more anchoring after a non-anchored guess, but without correcting for the 2-guess and perfect anchor confounds your results can't be properly interpreted.