When the user goes to /user/logout, they're logged out from Drupal, but the IdP is not involved at all.
Going to /user/logout is enough to reproduce.
I could give you my full configuration if that's any useful.
Single Logout is enabled of course.
Single Logout works with other webapplications, so it's not the IdP's fault.
It's also not a validation error: if I look at the access.log of my SimpleSamlPHP server, I see no request at all coming to the logout endpoint when I logout.
If I try to go to /saml/logout (which I find in the code, but nowhere in the UI), it tells me my user is unauthorized.
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Oh, by the way, the exact same configuration works fine on a very old Drupal7 installation I still have around, on which I'm using saml_sp 3.4.
I don't have a use case for Single Logout, therefore I have no way to test if it works. If you would like to provide a merge request I can certainly merge it in.