Yes, we can tag a split transaction in QuickBooks Self-Employed, art.
For now, we're receiving reports with other users not being able to save tags into a split transaction. We would like to inform you that our Engineering Team has already created a ticket about this issue.
While they do, I would advise you to get in touch with our QuickBooks Support. This way they can add your account information to the list of affected users and attach your case to the ongoing investigation. Any progress will be communicated via email.
Please be reminded that our Support Team is available from 6:00 AM until 6:00 PM on weekdays, and 6:00 AM till 3:00 PM on Saturdays. Here's how to contact us:
1. Click the Help (?) icon.

2. Choose Contact Us.

3. Enter a brief description of your situation in the What can we help you with? area, then click Let's talk.

4. You'll be presented with a few options for connecting with Intuit. Select Get a call.

In case you need help with categorizing your transactions in QuickBooks Self-Employed, you can refer to this article: Updates to expense categories in QuickBooks Self-Employed. This will help guide you on how to report your self-employed income and expenses.
I appreciate your patience while we're working on this. If you have any other issues or concerns, please let me know by dropping a comment below. I'm always here to help, art. Have a lovely week ahead!