Custom SPI getting 'unnamed module of loader' error on Keycloak 19 startup with Quarkus

We have a custom SPI adapter that loads fine with Keycloak 19 Wildfly. We are working on transition to Quarkus, but the same SPI adapter is throwing the following error:

ERROR [org.keycloak.quarkus.runtime.cli.ExecutionExceptionHandler] (main) ERROR: class cannot be cast to class ( and are in unnamed module of loader io.quarkus.bootstrap.runner.RunnerClassLoader @1c6b6478)

Any ideas what is causing this or how to solve it?