As of iOS 10.3 beta 2 keychain information will be deleted when an app is deleted. Keychain information used to persist between app uninstall and reinstalls but that was not intended behavior and was a side effect of implementation that people got used to.
You'll probably just have to get used to the new behavior unless they have a top secret keychain replacement in the pipeline. keychain data should still persist since 10.3 beta 2 across app uninstall / reinstall by using an app group that shares said keychain info and having another application in the app group installed; so it doesn't delete the keychain info. Also I would guess it doesn't delete items synchronized with iCloud from iCloud but just deletes the local copy. Just a guess though.
Here is a link to an apple developer forum on the issue:
https://forums.developer.apple.com/message/210531#210531