I know, another day another split feature request. It happens.
Today I was splitting a category Staff topic and while I did notice that the new topic had the category uncategorized, I left it uncategorized to see what would happen. My hope was that it being a category Staff that it would be considered sensitive and retain the category Staff. When checking the new topic after it was created it was category uncategorized. Yes I immediately changed it.
Needless to say or request, but when a category Staff topic is split, it should default to category Staff
Other feature request related to Split Topic:
Allow users to request topic split in formal manner
Duplicate post when splitting