Bug description:
As you can see in the linked imgur album below, a container that is not the currently focused one fills up faster than the other two non-focused containers. This always seems to be current container plus/minus 2 aka while the 1st container is focused the 3rd container fiils up faster, during the 2nd the 4th container fills faster and for the 3rd/4th the 1st/2nd containers respectively. This behavior isn’t mentioned anywhere in the dungeon compendium and therefore I would classify it as a bug.
Reproducability:
100%, it happens on every single attempt (not sure if this is a mythic difficulty exclusive bug or not). I watched some of the earliest kill videos of this boss and it was happening there aswell, so not a new bug either.
Proposed solution:
Either remove the faster fill rate on non-focused containers or document it in the dungeon journal.