Backpacks are portable storage items that keep their own contents on the item itself. Unlike the Ender Backpack, normal backpacks do not open vanilla Ender Chest storage.
Right-click a backpack while holding a single copy of it to open its storage. The first time a blank backpack is opened, that specific item gets its own personal inventory, so two backpacks of the same tier do not share contents.
/sf backpack to help retrieve lost contents.If you want portable access to your Ender Chest instead of private backpack storage, use an Ender Backpack.