Abnormal bitrate and quality drop
You can find all the informations here : https://forum.doom9.org/showthread.php?t=175662
Otherwise, short version : 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.