Hello there, Victor. Let me share some details about splitting transactions in QuickBooks Self-Employed (QBSE).
When assigning or splitting a transaction to the Mortgage home office category in QBSE, it will not reflect in your Profit & Loss and Tax Summary/Detail reports. To learn how Schedule C categories appear on reports, you can read this article: Schedule C and expense categories in QuickBooks Solopreneur and QBSE.
If you need to reflect those amounts as business expense, you'll have to choose other category. To ensure accuracy and compliance, I recommend consulting an accounting professional.
I'll keep an eye out for your reply. And if you have any other QBSE-related concerns, feel free to include them in the comments. We're here to assist you 24/7. Have a great day!