When a user logs in in iOS via the webview and select ‘save my credentials’ and than logs out again the cookie is not removed.
Calling login again will cause the webview to redirect directly to the stored logged in user preventing from ever selecting a other account.
If the user selects logout the cookie is the cache should be removed or the login screen should ask something like:
'Are you xxxxxx? Press Yes to continue or Login with another account'