When a lender initiates refinanceFull() or refinancePartial(), _remainingNewLender will be set to type(uint256).max (src/lib/loans/MultiSourceLoan.sol#L280), which indicates the lender will repay existing lenders.
However, even when _remainingNewLender is set to type(uint256).max, in _processOldTranche(), _remainingNewLender -= oldLenderDebt will still run. This is not meaningful.
src/lib/loans/MultiSourceLoan.sol#L665
In _processOldTranche(), add a check and only perform the subtraction when _remainingNewLender != type(uint256).max.
