I am encountering the same problem. The UI seems to allow multiple URLs and indices in "Requestable SSO URLs", which presumably correspond to AssertionConsumerServiceIndex and AssertionConsumerServiceURL, but the URL POSTed to is always the "Single sign on URL". I'm not sure why those are in the UI, since they seem to do nothing. Maybe a bug. I am trying to use multiple URLs so I don't need to make an app for every single SP I have.
FYI: I got this working by making sure my AuthnRequest was properly set up. I'm still not sure what the issue was, but it might have been that I needed to set the protocol binding to POST in my AuthnRequest. This works for me for both AssertionConsumerServiceIndex and AssertionConsumerServiceURL.