All jurists agree on the rule of loss: each partner bears loss exactly in proportion to the capital invested.
If A invested 40% and B invested 60%, a genuine loss is borne 40/60. The partners cannot validly agree that A will bear only 10% of the loss while owning 40% of the capital.
The practical test is to separate normal business loss from loss caused by negligence or misconduct. Genuine investment loss follows the relevant capital rule, while a party remains responsible for loss caused by its own fault.