Never heard of a Gleeson Six doing that, but, They only get about 1% of the prostate when they do a biopsy. It is not unusual for somebody with a 3+4 to find out after surgery that they had a higher Gleason score. In my case, it was a 4+3 after surgery, I know people that had 4+5.

It is possible they missed some things that are aggressive. Have you had an MRI? That can tell whether or not there are lesions that are more suspicious than Gleeson 6.