OAuth authorization-server metadata is unreachable at the RFC 8414 issuer-derived path when XWiki runs under a context path

The OIDC extension cannot really do anything about http://localhost:8080/.well-known/oauth-authorization-server/xwiki/oidc, XWiki in general cannot if it's configured with context path /xwiki/.

All you can do is configure the HTTP proxy to serve http://localhost:8080/xwiki/oidc/.well-known/oauth-authorization-server when http://localhost:8080/.well-known/oauth-authorization-server/xwiki/oidc is requested.