I figured out that when there is a EV adjustement (in my case shutter speed increase from 1/320 to 1/250) from the second last to the last image, this is not handled correctly.
There is no 2*/3* detection in this case.
I also see the problem I think... the last image is already marked as 1* keyframe denoting the end of the sequence and should be at the same time a 3* keyframe denoting the EV change for compensation.
So the Holy Grail Wizard is failing to do the exposure compensation for this last 2 images and just ramps the exposure from the last detected 2*/3* keyframes not taking the last EV change into account, which leads to a EV jump from the second last to the last image.

The workaround is easy, just drop the last frame.. but I thought I let you know, maybe there is a way to fix this.

Thanks for the note, but like you said, It normally just doesn't make much sense to make an adjustment between the second last and the last image ;-)