This issue was caused by Samsung’s “Nearby Device Scanning” feature. It is only present on Samsung devices (and not all of them). For affected users, Nearby Device Scanning will cause the app to take forever to see the spirometer if it does at all. If you can connect, it’ll probably disconnect at some point.
When you turn this feature off in the settings, the issue goes away and you can pair to the spirometer normally. https://www.techbone.net/samsung/user-manual/nearby-device-scanning