It could be that the cloack aloud her to inorder to protect Harry, the same way the Elder Wand wont hurt its master. This way the Cloack was helping Harry by allowing her to. The most likely is the Film makers forgot this and hey, its Warner Bros. It cant all be canon or perfect.
Firstly, Luna finding Harry is a contradiction to the book. In the book, it was Tonks who found Harry, and she removed the cloak by hand. Secondly, while it is possible that the cloak allowed itself to be removed to better serve its rightful master, Harry, it is more likely that Luna simply summoned up a gust of wind to blow the cloak away. While immune to the direct effects of spells, it isn't immune to things like hands or winds moving it.