When you press Ctrl+Shift+C and add a Child of Constraint, it is useless/unusable from start, 99.99% of the time.
You immediately have to go to the constraints panel and click Set Inverse to make it usable.
You also have to be told that that’s a required step and not doing so is why the constraint acts broken.
The only scenario I can think of where it works as desired (by anyone) from the start, is when your bone/object is at the World Center when the constraint is created, or a similar scenario when one or both object/bones don’t have any transforms.
This is an odd quirk native only to Blender (afaik) despite the fact many other programs having a similar constraint.
I specify the ctrl+shift+C menu but there’s no reason to not fix it for other instances like the regular panel.
The inverse matrix can auto update when you change targets, like the Reset Length does for the Stretch To constraint.