Mutiny 2 is based on the java.util.concurrent.Flow API instead of the Reactive Streams API. In other words, uni.convert().toPublisher() returns a java.util.concurrent.Flow.Publisher instead of an org.reactivestreams.Publisher. However, the ReactiveAdapterRegistry class adapts to Mutiny 1, which leads to runtime errors with Mutiny 2.

See https://smallrye.io/smallrye-mutiny/2.3.1/reference/migrating-to-mutiny-2/.