I'm not sure what you mean by the big white button, do you mean the disengage? There are 3 auto pilot engage buttons plus two flight director switches and an Auto throttle arm switch, which auto pilot button are you using, left, center or right? Are you using the flight director and A/T switches? Are you turning on and allowing enough time for the IRS to align? There has been some pretty big differences between Blk F and B, have you read through the manual or followed any pre flight checklists? How are you going about setting up the AP, can you describe this in some detail, aside from pressing the big white button? I'm afraid theres a little more to it then 1 button!

I'm not trying to be rude it just seems you may be missing some steps introduced by the new features.
If you do have a legitimate bug I sincerely apologize  ;)