why this IDL is not plain

[scoopySHI]

Hi, i have this Time.idl file similiar with ROS

module builtin_interfaces {
  module msg {
    struct Time {
      long sec;
      unsigned long nanosec;
    };
  };
};

and as i checked if it is plain using is_plain() it show its not. I think here only plain type are used, i wonder why this is not plain, becuz of its nested within module(namespace)?

fastddsgen -version output

openjdk version "11.0.26" 2025-01-21
OpenJDK Runtime Environment (build 11.0.26+4-post-Ubuntu-1ubuntu122.04)
OpenJDK 64-Bit Server VM (build 11.0.26+4-post-Ubuntu-1ubuntu122.04, mixed mode, sharing)
fastddsgen version 3.2.1

while for some reason i am using FastDDS 2.12.