did you make sure you ran the halyard through multiple times to make sure the twists were out of it?
The same thing can happen with regular dyneema but not as easily. What kind of 6mm dyneema did you order?
The core of the line you are using (I think it may be the same as mine maybe? who knows) would work and if you do that you probably don't need to order any 6mm dyneema. I find that the core is or any dyneema is a little too slippery to pull on extcept for the last few feet when dousing (I have mine tapered at the snuffer end). The dyneema will wear quicker at the cleat than covered line.