We are working on the different options of the span & split columns commands in InDesign CC 2017 on the different platforms at the same time, Windows and Mac.
With the 2 computers at the same time we are doing the tests:
1.- Create a big column and fill with placeholder text
2.- Apply Split 2 columns
3.- We have 2 columns of false text.
4.- In the first column we write 8 words, like a list.
5.- Apply span 4, to extend this in the first column.
Mac works well!
In windows expands the 8 words over two columns, no only one.
Maybe this is a bug, special windows.
Screen captures would be helpful so we can test to see what you're creating. You can attach them in the forum editor.
I tried to reproduce the same but was not able to do so. It would really be helpful if you can share a screen capture of your workflow, as suggested above by Steve, so that we can check it at our end.