I've worked on (non-Atlassian) SSO projects where the provider used the referrer to send the client to the page-after-logout (and occasionally page-after-login) if they weren't set as parameters in some circumstances.
Here's a reference to a F5 device providing SAML SSO services and having a similar issue:
https://www.devcentral.f5.com/s/question/0D51T00007npfjw/chr...