I have encountered this issue three times now where one of your companions somehow unlinked from the group and you have to link them back again. The first time was with Shadowheart not jumping down from the cliff where you get the spider pouch. The second time was with Astarion after you fight the harpies at the beach. When I went back into the Druid Grove, he wasn't following and found him standing on the beach. Just the same with Shadowheart, he was unchained from the group. The third time was once again with Shadowheart when I was fighting the knoll hoard in the cave near The Risen Road. I notice in the middle of battle that she was not in the group but standing at the entrance of the cave, having somehow been unlinked.