Sigbert and Godwin bug

The duo boss fight Sigbert and Godwin in Holten, in Interlude act, can be fought 1 at a time by staying at ones location during the bosses spawning. The other one will never notice you.

I am starting to notice this bug appearing as a pattern in every boss. This is now the third boss I have reported the same bug about, where the boss does not aggro you or loses aggro if you just walk to the other end of the arena so that the boss (or one of two bosses in a duo fight) is offscreen.
Happened with Krutog in Act 4 and the duo witches in Interlude also.

So it's a general bug that probably applies to all bosses? Needs fixing.

I noticed that the Miller can lose aggro on you in the same way, before Clearfell when starting the campaign, if he runs to one corner of the map (by the checkpoint) and you run to the other side.