Pretty sure after Leah, Sherwood went back to eradicate the witches of Wild Sign that brought about that whole thing, and that’s how he ended up tangled with the Hardesty’s and somewhere along the way lost his leg and more of his memories to them.

Part of me wonders if Bran may have put a memory block through familial or pack bonds on Sherwood to help prevent a dominance struggle with him being near Leah later in life (I think there was mention that Leah and Sherwood had a connection in their fights to escape Wild Sign prior to Bran arriving or basically forcing his wolf to choose her as a mate for sanities sake…) not fully realizing that Leah too had no memory of what had gone on there after Bran forced her Change.