Here's a minimal example to illustrate the problem:

import java.util.concurrent.Callable;
import picocli.CommandLine;
import picocli.CommandLine.ArgGroup;
import picocli.CommandLine.Command;
import picocli.CommandLine.Option;

@Command(name = "multiplicity")
class Multiplicity implements Callable<Integer> {

  @ArgGroup(exclusive = false, multiplicity = "1")
  private RequiredGroup requiredGroup;

  static class RequiredGroup {
    @Option(names = "--option_one")
    private String optionOne;

    @Option(names = "--option_two")
    private String optionTwo;
  }

  @Override
  public Integer call() throws Exception {
    return 0;
  }

  public static void main(String... args) {
    int exitCode = new CommandLine(new Multiplicity()).execute(args);
    System.exit(exitCode);
  }
}

With this example, I would expect that the command without any options (i.e. multiplicity), would throw a MissingParameterException. However, in the current version (4.7.4) the command executes and the call() method is executed. Note, that the requiredGroup object is null.

On a much older version of picocli (4.1.4), running this command works as expected. It throws a MissingParameterException with the text Error: Missing required argument(s):.

I am not sure exactly where the regression happened or if this is even a regression? From my reading of the docs, it does seem like one. In the ArgGroup documentation:

define a set of arguments that must co-occur. Set exclusive = false to define a group of options and positional parameters that must always be specified together. Picocli will throw a MissingParameterException if not all the options and positional parameters in a co-occurring group are specified together.

Groups may be optional (multiplicity = "0..1"), required (multiplicity = "1")

When the parent group is required, at least one subgroup must be present.