I uploaded an iOS enterprise build using App-Distribution feature of Firebase Console and shared with my testers. We are able to download it on all devices except on iPhone X. The error shows "os version not supported". On click of this error, it displays details that "The developer needs to upload a build that supports iOS 10.15". Please check screenshots for reference.
Please note that iPhone X have iOS 13.3. I suspect not be an iOS version issue, because I downloaded app on my personal iPhone XR which has iOS 13.3.