This is interesting. I did a batch the way i did it before, which i know is wrong, just to see. The fermentation temperature varied between 60 and 80 degrees (night and day). I wont do that again, but i was curious.
I did do a diacytel rest but maybe should have done it a few days longer. I read that highly flocculant yeast are not as good as absorbing diacytel as well, but the yeast i used (WLP400 - Belgain wit ale) says it is low flocculating.
Maybe a higher fermentation temperature causes the yeast to speed up too much and die off before they get a chance to mellow and absorb some of the butter?