DateTimeFormatterBuilder Mapped-values do not work for JapaneseDate

DateTimeFormatterBuilder Mapped-values do not work for JapaneseDate. They
work for LocalDate.

The problem occurs with both toFormatter(Locale.US) and toFormatter(Locale.JAPANESE).


TESTCASE
--------
import java.text.*;
import java.util.*;
import java.time.*;
import java.time.format.*;
import java.time.temporal.*;
import java.time.chrono.*;

public class FormatterBuilderTest_ja_jp{
    public static void main(String[] args) {
        String str;
        DateTimeFormatterBuilder builder = new DateTimeFormatterBuilder();
        Map<Long, String> yearMap = new HashMap<>();
        yearMap.put(1L, "1st");

        Map<Long, String> monthMap = new HashMap<>();
        for (long l = 1; l <= 12; l++) {
            monthMap.put(l, l+"gatsu");
        }

        builder.appendText(ChronoField.ERA, TextStyle.FULL);
        builder.appendLiteral(' ');
        builder.appendText(ChronoField.YEAR_OF_ERA, yearMap);
        builder.appendLiteral('-');
        builder.appendText(ChronoField.MONTH_OF_YEAR, monthMap);

        DateTimeFormatter formatter = builder.toFormatter(Locale.US);
        // DateTimeFormatter formatter =
builder.toFormatter(Locale.JAPANESE);

        JapaneseDate date = JapaneseDate.of(JapaneseEra.HEISEI, 1, 4, 1);
        str = date.format(formatter);
        System.out.println(str);

        LocalDate local = LocalDate.of( 1, 4, 1);
        str = local.format(formatter);
        System.out.println(str);
    }
}

REPRODUCTION INSTRUCTIONS
-------------------------
Compile and run the testcase.

Observed output:
> Heisei 1-April <== Problem
> Anno Domini 1st-4gatsu

Expected output:
> Heisei 1st-4gatsu
> Anno Domini 1st-4gatsu