Hmm, I have never gotten this issue before. It's not something I've implemented, it can very well be Itch.io's internal system. I suggest writing to itch directly for support. I'll look into it myself as well. Let me know if the issue persists.
Also, feel free to write directly to me via mail (PenzillaDesign@gmail.com); I'll be happy to sell the assets directly circumventing the issue :)