The staff could help you out if you have proof or a screenshot, but keep in mind when it says clear lag please do not drop anything you need on the ground or break it.
I'm sorry you lost your items in your shulker box. Unfortunately, without proof (as Xsuma mentioned), any items lost through ClearLag is considered "Player Error" and may not be refunded. Once again, I do apologize for the inconvenience. If you have any other problems, please feel free to contact us and we will help you out as soon as possible. Stay safe and healthy.