Let's see if I got this right, this is me interpreting what I've seen here.
The BossBrain actor has no defined Radius value, making it default to a Radius value of 20, when the value should actually be 16.
This makes no appreciable difference in the vast majority of use cases, but it did cause problems in a level where the BossBrain was supposed to move through a 32 unit wide conveyor path, causing it to get stuck and subsequently get telefragged by the decoration coming in behind it.
In DSDA this wasn't happening.
It occurs to me that this could change how splash damage interacts with the BossBrain actor though, and I don't have a particularly good solution for that.
As said, this is largely a very uncommon issue due to very few levels depending on the Radius of the BossBrain to function (in fact this may be the first example ever), and including a basic Decorate patch would be a trivial fix, but I figure it should be reported at any rate.