So this just started recently, and isn't limited to just the Split feature but I can't remember what other features I've selected when it's done this before*. Anyway, I have a part, click on Split and it asks me to select a new document. I can either choose a template or hit cancel. The result is the same; the dialog box closes and the feature operates normally as if the dialog box never popped up.
Didn't always do this and not sure why it started doing this recently.
*Edit: "Save Bodies" will prompt the new document dialog box to pop up, but it doesn't matter what i click, or hit cancel, either.