It's a bug in the CPU firmware, which can not be updated. The PRO-96's with CPU v 1.5 corrected this. It has to do with how the CPU interprets numbers in binary format. I think it was Loumagg that had a better explanation of the problem.
Using Lou's formula, you can figure out the good talk-group that is allowing the offending blocked talk-group to get though. If you can live without the good talk-group, remove it, and the related offending talk-group will be stopped.