(please don't hesitate to correct me if I'm wrong)
Say there are 3 customers at a single bank.
Customer 1 = Investor, has 100 cash and deposits it into a bank account
Customer 2 = Borrower, borrows 100 from the bank
Customer 3 = Restaurant, provides a service for Borrower. Borrower pays a 100, and Restaurant deposits it to it's bank account.
This is how the "sovereign money" travels: Investor -> Borrower -> Restaurant
But the customers see the following account balances
Investor : 100
Borrower : -100 (owes the bank 100)
Restaurant : 100
If now Investor and Restaurant both want to withdraw their money the Bank would be in trouble. The bank only has 100 "sovereign money" on their books. Investor and Restaurant won't care who owes the bank, they want their money.
But the solution for this would be easy, Investor must be given the choice if he wants to allow/disallow the bank to loan out his deposit. Similarly how it works with long positions at a brokerage firm.