When dispensing shulker boxes, items within shulker boxes get expelled from it into the next container filling hoppers/containers connected to hoppers. Not sure if this is intentional. Videos have been attached
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Thank you for providing videos. They show hoppers, hopper minecarts, shulker boxes, and dispensers working as intended.
You are dispensing the shulker box directly above a hopper minecart. The hopper minecart pulls the items out of the shulker box, and the hopper under the hopper minecart pulls the items out of the hopper minecart and pushes them into the dispenser.
You might want to look on the wiki, search for a video, or ask in a Minecraft community for help building a shulker box unloader.
Do you have the hopper minecart on a powered activator rail? If not, then it might be what is sucking all the items out, as it isn't locked.