You'll want to perform these steps to see if this is just a browser error. First, use an incognito window and split the transactions from there. Also, let's clear the cache since it can cause issues in QuickBooks Online.
Lastly, make sure that the browser is updated. You can use a different one if there's available.
Post again here if you have other questions. Stay safe!
I appreciate you for getting back to us here in the Community. Let me provide some information about what to do next to split transactions successfully in the Banking section.
Since the problem persists after performing the necessary troubleshooting, I suggest you proceed with contacting our QuickBooks Support Team. You can use the Start a chat feature from the Help menu for them to securely access your account and assist you further with the banking issue.
Here's how:
Click Help in the upper-right corner.
Select Contact Us.
Enter your specific concern in the What can we help you with? box.