Why does post_logout_redirect_uri need to match domain of one of the 'redirect_uri' values when the OIDC spec doesn't require it?

[mig5]

Hi,

I tried setting an OIDC client to have a post_logout_redirect_uri of https://foobar.example.com, whilst the redirect_uri (e.g post login redirect uri) is https://whatever.example.com.

I got an error:

message:invalid_client_metadata reason:post_logout_redirect_uri "https://foobar.example.com" must match the domain, port, scheme of at least one of the registered redirect URIs but did not

I already understand that for login, the callback URL must match one of the redirect_uris in the OIDC client.

But as far as I can tell, it is not mandated in the OIDC spec that the post_logout_redirect_uri domain needs to match the post login redirect uri domain.

Can I get confirmation that Ory have enforced this requirement in Hydra nonetheless? Is that what I'm seeing at https://github.com/ory/hydra/blob/master/client/validator.go#L197-L203 (I'm not too good with Golang).

Is it documented somewhere (that I haven't found?)