I had the same problem this morning, I think they are in mid progress in switching the split ship process or something... there was nothing in the update blog post about it.
When I was able to get it to work (I think I manually went into the order and split it, instead of clicking split-ship in the "other-actions" menu") It was splitting the order into two individual orders, with the same order number.
I'll try and make more notes if I get more split shipments tomorrow, was in a bit of a rush to get things done today.
Unless a Mod could be so kind as to address the issue.
Hey there everyone!
Thank you for your patience.
This behavior was reported and investigated, and, I am happy to report, it has since been resolved. The senior support team has rolled out a fix that should allow you to once again split orders!
If you find that this ability is still missing from your account, please reach out to the support team right away so that we can troubleshoot that process with you directly in your account.