I can't say is it bug, but even if it is bug, it's never fixed with any firmware. Drive really burns at 2.4x speed.
One interesting thing: 2 of my 3 BD-RE discs which were burned at 2.4x were unreadable on my next BluRay burner (LG BP40NS20). It reads them as blanks.
When I put them back to BH10LS30, it reads them normally. I reuse these BD-RE later on my next burner.