help for subcommand not showing

My yargs structure shows top-level help when I call

node yargs-issue.js --help

generate plain html from json

Commands:
  yargs-issue.js plain <json>  generate plain html from json           [default]
  yargs-issue.js audio <json>  generate html with audio support

Positionals:
  json  the json file as found in the ../JSON/ directory

Options:
  --version           Show version number                              [boolean]
  --help              Show help                                        [boolean]
  --speakers, -s      speakers                                [array] [required]
  --release-date, -r  release date, e.g. 'November 25, 2019'          [required]

But when I try to invoke help from a subcommand:

./yargs-issue.js audio --help

the options for that subcommand are not shown. Instead I get the exact same output as before.
However, if I call the audio subcommand incorrectly by omitting a required option, I get the desired information about the subcommand-specific options:

node yargs-issue.js audio
yargs-issue.js audio <json>

generate html with audio support

Positionals:
  json  the json file as found in the ../JSON/ directory

Options:
  --version           Show version number                              [boolean]
  --help              Show help                                        [boolean]
  --speakers, -s      speakers                                [array] [required]
  --release-date, -r  release date, e.g. 'November 25, 2019'          [required]
  --audio-file, -a    audio file
  --audio-offset, -o  audio file offset when speech starts in seconds [float]
                                                                      [required]

Not enough non-option arguments: got 0, need at least 1

This is the source code of yargs-issue.js

const argv = require('yargs')
  .command(['plain <json>', '$0'], 'generate plain html from json', yargs => {
    yargs
    .positional('json', {
      describe: 'the json file as found in the ../JSON/ directory',
      nargs: 1,
      global: true, // used for both plain and audio commands
    })
    .option('speakers', {
      alias: 's',
      describe: 'speakers',
      demandOption: true,
      array: true,
      global: true,
    })
    .option('release-date', { // (will be renamed to releaseDate)
      alias: 'r',
      describe: 'release date, e.g. \'November 25, 2019\'',
      demandOption: true,
      nargs: 1,
      global: true,
    })
    .command('audio <json>', 'generate html with audio support', 
      yargs => { // builder
        return yargs
          .option('audio-file', {
            alias: 'a',
            describe: 'audio file',
            nargs: 1,
          })
          .option('audio-offset', { 
            alias: 'o',
            describe: 'audio file offset when speech starts in seconds [float]',
            demandOption: true,
            nargs: 1,
          })
      },
    )
  })
  .strict()
  .help()
  .argv

Start with the command definitions and help configuration in yargs-issue.js. Reproduce the behavior with ./yargs-issue.js audio --help and compare it with the missing-option output from node yargs-issue.js audio. Done means the audio subcommand displays its own options when invoked with --help, without changing the top-level help output.