I’m not totally sure I understand your setup. Can you show me some screen shots of the hierarchy or any relevant blueprints.
If I’m understanding correctly, you’re syncing three different things: the helicopter, the attachment point, and the container itself. Make sure you’ve got a Smooth Sync component for each of those, and that Replicate Movement is off for all of them.
The attachment / detachment needs to happen everywhere, so it should be multicast.
You probably also want to enable the “Sync Ownership Change” option as the owner is likely changing from the server to the player when attached and back when detached.