Hi there, @ianclapham
I'm here to help you enter split expense transactions.
If you're on the review tab page, please make sure to click the categorize option.
Please see the screenshot below for your reference.
Here's an article for a reference: Download, match, and categorise your bank transactions in QuickBooks Online
Please feel free to get back to us if ever you have any other questions or updates. I'll be looking forward to hearing back from you.