I have no idea! That's all a new one on me.
Contact support@itch.io and take a free Community Copy in the meantime.
An update! I cancelled the PDF-only order through BK, then pre-ordered the Printed version, and the order email has the pdf downloads available now! Maybe there was a glitch before, but also maybe the PDF-only version has the downloads locked until the project is complete?
For me, it's resolved. But just wanted to let you know in case anyone else has issues :)