I can't get the Child Of constraint to actually work

I'm using Blender 2.92 and Library Overrides instead of proxies. I suspect this might be the reason why I'm having problems getting the Child Of constraint to work. The issue is that while I can set up and animate its parameters correctly, the magazine bone doesn't actually follow the hand bone, it just stays in place as if I couldn't read its transform data correctly. When changing the influence, the Child Of constraint that makes it follow the root bone does work, meaning that the magazine is detached from the gun, but the other Child Of that makes is follow the left hand doesn't. Something that reinforces this theory is that, when using the eyedrop in the Target parameter of the constraint, I can't select the Library Override rig (the player arms, in my case). The only way to set it as a target, is to type its name and select it from the dropdown.