I think what's unclear is what "fall off" means?
It means does not pass test the board must pass for me to accept it.

Do you let your contract manufacturer make engineering decisions as to which parts are responsible for failures ?
The contract manufacturerer has been given a test the board must pass, if it does not pass, the CM is responsible for making it pass (repair or replace).

Erik

PS the extract is from an e-mail asking if substituting a better part is OK and am I willing to pay a few more cents to compensate for the price differential.