I don't know, man. Kind of wondered the same thing myself. I cut mine a few inches shorter to remove some of the extra slack, and I'm wondering if there is too much tension on the tube now to let it float loosely. Only thing I can think of.
My guess is it isn't floating perfectly, and pulling air from the headspace too. This happens with my torpedo floating dip tube. Towards the end of the pour, seems like the dip tube is no longer in the liquid. I just give it a little shake and it's usually ok.