When you pressed L2, does it show a blank portrait slot-holder where she should be? Or is she just completely gone from your panel?
I checked
BG3 Wiki, and another way players can lose companions is if she had Very Low approval of your character:
Value Approval Description
-49 � -40 Very Low Relationship is in danger. Character is likely to leave party if they disagree with another decision.
-39 � -20 Low Relationship is bad. Character may leave party if approval gets worse.
-19 � 20Neutral Relationship is neutral. Character has no strong feelings towards avatar.
21 � 40 Medium Relationship is fair. Character's trust in avatar is variable.
41 � 60 High Relationship is good. Character approves of avatar.
61 � 80 Very High Relationship is very good. Character is happy with and easily convinced by avatar.
81 � 100 Exceptional Relationship is exceptional. Character is thrilled with and very easily convinced by avatar.
Low Approval
If a companion's approval drops to -50 or lower, they will leave the party permanently, taking with them anything they had equipped. The contents of their inventory including story items will be returned to the player character in a backpack.
Companions will give two warnings (once at -25 and once at -40) before deciding to leave.
Check your inventory, and see if you have Shadowheart's backpack. If she returned your items to you, she has left your party.
