Currently recurring business income does not show up in the spending plan. Planned recurring income in business categories or outstanding invoices should show up in the spending plan for the month. One could set an option for whether business transactions, or which businesses' transactions, show up in the spending plan.
This has been described as a bug: https://community.simplifimoney.com/discussion/comment/47892
There is a feature request for separate business spending plans. Any spending plan should just include whichever combination or personal or businesses the user selects