Might have been a lack of clarity on my part. When you split, you are actually choosing "what goes first". The issue workflow-wise today is primarily a lack of tooling designed to represent this process visually in an easy way.
Except…when you’re actually splitting an existing commit with interactive rebase, don’t Git tools make you unstage changes that should go second? I see how it’s backwards and annoying compared to “git add -p” for a new commit, but they seem equally bad for splitting existing ones.