When splitting a US there is no error, the procedure is performed. However, we observe that a new US number is created and remains in the sprint we are working on and the old US number is automatically sent to the future sprint. In other words, the system is inverting the numbering of the User Stories.
This is currently working as designed.
This behavior is documented in our help page link below (in the "Split an Unfinished Story" section):
and it is also documented on our "Split Story" page: