Stalwart Shield-Bearers continually checks which creatures you control have defender and gives the bonus only to them. If an effect causes a creature with defender to lose defender (as Shoal Serpent does), Stalwart Shield-Bearers stops giving it the bonus for as long as it doesn’t have defender. On the other hand, if a spell or ability causes a creature to be able to attack as though it didn’t have defender (as Warmonger’s Chariot does), Stalwart Shield-Bearers continues to give it the bonus because it never actually loses the defender ability.