Abnormal bitrate and quality drop

[mp3dom]

Using x264 on blurays (high bitrates and vbv)

Under certain unknown circumstances, there are significant bitrates and very visible quality drop on scene changes. Generally, it appears more often when the previous scene is quite complex with some particles effects (snow falling, fire crackles, and so on...). The bitrate drop is very pronunced, it can be even 5x lower than expected.
The problem propagates for no more than a gop (on bluray it translates to 1 second), infact, the *same* scene, after that second, "regain" its bitrate and continue with good quality. I'm not even able to fix it with the use of zones, because with zones I can fix that scene a bit (albeit x264 totally ignore my bitrate multiplier but still it raise the bitrate in a decent way) but the problem is then "shifted", and just after the zone ends, the same bitrate drop appear.

This is an example:



As you can see here, after a scene change (new scene), the bitrate drops enormously (notice the next six B/P frames with their very low size). Then at the next gop (same scene I frame), the bitrate returns to its right value and so the quality.

The quality in that zone is really low, with lots of compression artefacts... and the average bitrate of the stream should be 33 Mbps.

Also, I would like to point out that it's not happening with a particular x264 release... I already encoutered this type of problem with elder x264 revisions. Depending on the footage, on a general 2 hrs movie this may happens 3-4 times. With the movie I'm working on now (action anime), it's happening 15 times