I had this same issue. It happened after I had some characters stealthed when first encountering the Grove/goblin fight. Shadowheart stopped following. I tried unlinking/relinking. Group linking/unlinking. I tried going to camp and swapping her out, bringing her back. The other companions would follow fine if Shadowheart was leading. Unfortunately, I don't think I can keep going through this playthrough (trying out the bard) if this is going to keep happening.
