That's standard behaviour in every analytics database that talks to kafka that I can think of. Kafka doesn't really encourage ACK/NACK type processing patterns. Accepting an event usually means the consumer has successfully read it and staged it for whatever is meant to happen next, not that the operation is completed.
Now if it were possible for it to accept an event from Kafka but not guarantee that the event will eventually make it into the materialized view but may be lost, that'd be a problem.
Now if it were possible for it to accept an event from Kafka but not guarantee that the event will eventually make it into the materialized view but may be lost, that'd be a problem.