A DESCRIPTION OF THE PROBLEM :
I have tested New Era for Japanese Imperial Calendar.
In Java 8, the letter "G" indicates a short or abbreviated form like "H" (for Heisei Era).
But, after Java 9 (through 12-ea), the letter "G" indicates the full form like "平成".

REGRESSION : Last worked in version 8u192

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Execute below source code with JShell

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
H31.01.04
ACTUAL -
平成31.01.04

---------- BEGIN SOURCE ----------
import java.text.*;
Locale locale = new Locale("ja", "JP", "JP");
DateFormat alphabetFormat = new SimpleDateFormat("Gyy.MM.dd", locale);
Calendar now = Calendar.getInstance(locale);
System.out.println(alphabetFormat.format(now.getTime()));

---------- END SOURCE ----------

FREQUENCY : always