Yes, a backend can trust more than one issuer, but that should be an explicit multi-issuer trust configuration—not “decode the JWT, read iss, then accept whatever keys that URL advertises.” The unverified iss value may only select an entry from a fixed allow-list. The backend must then validate the signature with that issuer's configured JWKS and validate the exact iss, the API's aud, expiry/not-before, and the scopes/roles used for authorization.

For this architecture I would choose between these models:

1. One realm, multiple clients/resources when all stacks are in the same security domain. Keycloak realms are intentionally isolated; clients are the normal boundary for multiple applications inside one realm. Give every API its own client/audience and map only the roles it needs. This is the simplest model for a user that should call several APIs.

2. Keep the realms isolated and explicitly add the conglomerate realm as a trusted issuer to each target API. Each API then needs a local authorization mapping for central identities/claims. A valid signature proves who issued the token; it does not automatically grant that central user permissions in the API's local domain.

3. Use a confidential BFF/gateway and token exchange when the frontend needs tokens tailored to different APIs. The browser sends its central token to the BFF; the BFF exchanges/downscopes it to a token whose aud is the target API, then calls that API. Keycloak's supported standard token exchange is designed for changing the target client/audience within a realm. For cross-realm/domain chaining, use Keycloak's documented identity-brokering/chaining pattern rather than forwarding one broad browser token everywhere.

Whichever model you choose, avoid validating only the signature and issuer. A token issued for frontend/client A should not be accepted by backend B unless B is an intended audience. Also avoid putting all target audiences and privileges into one long-lived “universal” browser token; per-API, short-lived, least-privilege tokens limit the blast radius.

So the blue-arrow design is plausible. If this is one organization with one user population, I would normally consolidate the applications into one realm and model the stacks as clients. If the realm separation represents real tenant/security boundaries, keep it and use an allow-listed multi-issuer validator or a BFF/token-chaining design, plus explicit claim-to-local-permission mapping in every backend.

If this answers the architecture question, please mark it as the accepted answer so future readers can find the security trade-offs quickly.