As per @Remy 's comment here, I’ve enabled DEBUG at the root level on the command line, and I can see error messages such as:
2023-03-16 09:54:37,958 [grpc-nio-worker-ELG-3-5] DEBUG io.grpc.netty.NettyServerHandler
So perhaps It’s just the logger name that is incorrect (i.e. io.grpc.netty instead of io.netty) [edit: no, that didn’t help]