Observed in WildFly 28.0.0.Final (Core 20.0.1.Final). Suppose you define a YAML config like so and pass it at startup using -y:

wildfly-configuration:
  system-propety:
    jboss.socket.binding.port-offset:
      value: 10000 

Note the typo: propety instead of propeRty. Despite the error, WildFly boots normally and there is nothing in the log about the problem.

I've classified this as a bug because the document in wildfly-proposals says it is a hard requirement for this to fail:

If the YAML is incorrect: like defining an non-existing attribute or child resource we should fail.

The PR for that document (#384) was never merged though, so I don't know if the thinking on this changed later. Personally, I agree with what the document says. An invalid XML config causes startup to fail, so invalid YAML should do the same. Or at least log some errors so the user knows there's a problem.