Here is the comment of Theodoret of Cyrus in his commentary on Jeremiah.
“By Ephraim he refers not to the son of Joseph but to the ten tribes; Reuben was the firstborn among them, and Simeon the second, but the ten tribes were called Ephraim since they built the palace in the portion of Ephraim. And just as today we are all called Romans after the city of the Romans, so at that time the ten tribes were named after Ephraim.”
Here is the comment by St. Jerome from his commentary on Jeremiah.
“Scripture speaks of Ephraim as a type of the people gathered in from the nations. For the younger son of Joseph stole the birthright from Manasseh, who was the firstborn by nature. But in the mystery of the cross Jacob’s hands crossed, and he who stood at Jacob’s left hand received the blessing of his right hand, and he who stood at Jacob’s right hand was blessed by the left and was reduced to the second rank. Just as Jacob stole the birthright from Esau, Ephraim stole it from Manasseh. Thus, all the people of the ten tribes are called Ephraim, since Jeroboam the son of Nebat, who was from this tribe, was the first to rule in Samaria.”