I'm not very surprised, there. Carapils adds more unfermentable sugars to the wort, rather than protein, and I understand that protein content is much more important to head formation and retention than unfermentables. The albumin, other proteins, and peptides create a matrix that the saccharides can supplement or just fill in, but they don't hold up like the proteins.
Just look at sea foam. That's protein, not sugar.
TL
So are unfermentables more about mouthfeel than head retention? I've used carapils in my last few batches and TBH, I can't tell the difference.