You probably have the mesh of your character set to:
Mesh->MeshComponentUpdateFlag = EMeshComponentUpdateFlag::AlwaysTickPose;
(This is the default for a Character).
This means that while the mesh is still ticked, the bones are however not evaluated while not rendered.
You want to set it to this instead:
AlwaysTickPoseAndRefreshBones
This will always tick and refresh bone transforms.
-Laurent