Sitecore ItemProvider does not provide 100% coverage for manipulating data behavior. You potentially need to implement both an ItemProvider and a GetItemCommand, or just the latter.

ItemProviderScenarios.cs

//ItemProvider works via GetItem override
Sitecore.Context.Database.Items[Sitecore.Data.ID.Parse("{110D559F-DEA5-42EA-9C1C-8A5DF7E70EF9}")].Children;
 
//ItemProvider works via GetChildren override
Sitecore.Data.Items.Item[] items = Sitecore.Context.Database.SelectItems("/sitecore/content/Home/*[@__isFolder='1']");
 
//ItemProvider doesn’t work, never called
items = Sitecore.Context.Database.SelectItems("/sitecore/content/Home/*");
 
//ItemProvider doesn’t work, never called
Sitecore.Data.Items.Item item = Sitecore.Context.Database.SelectSingleItem("/sitecore/content/Home");