Some from Manasseh defected to David when he went with the Philistines to fight against Saul. (They did not help the Philistines because the Philistine rulers consulted and sent David away, saying, “It will cost us our heads if he defects to his master Saul.”)
Jewish Publication Society of America Version (JPS1917) - 1 Chronicles 12:19 >(12-20) Of Manasseh also there fell away some to David, when he came with the Philistines against Saul to battle, but they helped them not; for the lords of the Philistines upon advisement sent him away, saying: 'He will fall away to his master Saul to the jeopardy of our heads.'