1080p video partly pixled after rendering
It honestly looks like something got corrupted with your export.  Have you tried encoding it more than once?  12 megabits per second with a 24mpbs peak should be more than sufficient for high quality 1080p video.  If anything, it may be excessive.  The quality you are getting looks more like what I'd expect at sub-1 mbps video for this quality level.  As an aside, it is worth noting that 2 pass will produce better results, thought it will extend encoding time.

One other thought is to try watching it in more than one viewer.  It is possible that the viewer you are using is struggling to decode the high data rate fast enough which could also result in the block issues you are seeing.  Basically, it looks like some of the blocks in the H.264 file are not encoding or decoding properly and that either means file corruption or insufficient decoding speed.

I guess to rule out data rates, you could try increasing it to 30mbps with 45mpbs peaks.  It shouldn't have to be that high (that's the high end of bluray data rates), but it should rule out the data rate entirely.

Update: From some additional reading, I would try turning on 2-pass and turning on the deblocking filter.  It supposedly should help.