SimplSAMLphp, when used as a proxy, does not properly propagate SLO requests from upstream Identity Providers. Received on it's SP endpoint they will simply terminate the SP session.

So in a given setup:

A SLO Request from a Our SP will be propagated to the Their SPs, but a SLO Request from a Their SP will not be propagated to the Our SPs.

See: https://groups.google.com/forum/#!topic/simplesamlphp/4Bm42hYfGjI