That's the risk with beta's.
But I'm sure rackspace will appreciate it if you contact them and tell them their app is inconsistent with the next version of iOS. Then they can update it before 11.3 is being released. Maybe they can even give you the new version of their app to test.
I own an iPhone 7 that was earlier running iOS 10. Today I updated to latest iOS 11.3 Beta but now whenever I am opening the Rackspace app it is only displaying a white blank screen on it's startup and simply closing itself after few seconds of non-responsiveness. How do I fix it??