ADDITIONAL SYSTEM INFORMATION :
Tested Windows 10 with Oracle JDK22, OpenJDK22, OpenJDK17

A DESCRIPTION OF THE PROBLEM :
Most documentation suggests that date format MMM is a 3 letter version of the month. This is not the case in English :- JDK returns 11 of the months as 3 characters, but September as 4 letters: "Sept". I cannot imagine why English MMM uses "Sept" for September. This makes exchanging dates in applications unreliable - reading and formatting dates with month abbreviation needs to pre-process text by String.replace("Sept","Sep") one way or the other.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run attached code example. Output will show MMM month format for UK and US and tries to parse a date in format "dd MMM yyyy"

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
There should not be any exception and it should print "MMM#9 UK=Sep US=Sep"
MMM#1 UK=Jan US=Jan
MMM#2 UK=Feb US=Feb
MMM#3 UK=Mar US=Mar
MMM#4 UK=Apr US=Apr
MMM#5 UK=May US=May
MMM#6 UK=Jun US=Jun
MMM#7 UK=Jul US=Jul
MMM#8 UK=Aug US=Aug
MMM#9 UK=Sep US=Sep
MMM#10 UK=Oct US=Oct
MMM#11 UK=Nov US=Nov
MMM#12 UK=Dec US=Dec
str1=20 Sept 23
str2=20 Sep 23
dt2=2023-09-20
dt1=2023-09-20

ACTUAL -
MMM#1 UK=Jan US=Jan
MMM#2 UK=Feb US=Feb
MMM#3 UK=Mar US=Mar
MMM#4 UK=Apr US=Apr
MMM#5 UK=May US=May
MMM#6 UK=Jun US=Jun
MMM#7 UK=Jul US=Jul
MMM#8 UK=Aug US=Aug
MMM#9 UK=Sept US=Sep
MMM#10 UK=Oct US=Oct
MMM#11 UK=Nov US=Nov
MMM#12 UK=Dec US=Dec
str1=20 Sept 23
str2=20 Sep 23
dt2=2023-09-20
Exception in thread "main" java.time.format.DateTimeParseException: Text '20 Sep 23' could not be parsed at index 3
        at java.base/java.time.format.DateTimeFormatter.parseResolved0(DateTimeFormatter.java:2108)
        at java.base/java.time.format.DateTimeFormatter.parse(DateTimeFormatter.java:2010)
        at java.base/java.time.LocalDate.parse(LocalDate.java:435)
        at DateProblem.main(DateProblem.java:52)

---------- BEGIN SOURCE ----------
/**
 * UK date formats with MMM are 3 letters long for all months except September 
 */
public class DateProblem
{
    public static void main(String... args)
    {
        final ZoneId gmt = ZoneId.of("GMT");

        final DateTimeFormatter fmtMMM = DateTimeFormatter.ofPattern("MMM").withZone(gmt);
        final DateTimeFormatter fmtMMM1 = fmtMMM.withLocale(Locale.UK);
        final DateTimeFormatter fmtMMM2 = fmtMMM.withLocale(Locale.US);

        // For me: Locale.getDefault().equals(Locale.UK) is true
        // System.out.println("UK? "+Locale.getDefault().equals(Locale.UK));

        for (int i = 1; i <= 12 ; i++)
        {
            LocalDate d = LocalDate.of(2024, i, 28);
            System.out.println("MMM#"+i+" UK="+fmtMMM1.format(d)+" US="+fmtMMM2.format(d));
        }

        // Note: RFC_1123_DATE_TIME does use "Sep"
//        DateTimeFormatter rfc1123 = DateTimeFormatter.RFC_1123_DATE_TIME.withLocale(Locale.UK);
//        for (int i = 1; i <= 12 ; i++)
//        {
//            ZonedDateTime dt = LocalDateTime.of(2024, i, 28, 12, 34, 56).atZone(gmt);
//            System.out.println("RFC_1123_DATE_TIME#"+i+" is "+rfc1123.format(dt));
//        }

        DateTimeFormatter ddMMMyyyy = DateTimeFormatter.ofPattern("dd MMM yy").withZone(gmt);
        DateTimeFormatter ddMMMyyyy1 = ddMMMyyyy.withLocale(Locale.UK);
        DateTimeFormatter ddMMMyyyy2 = ddMMMyyyy.withLocale(Locale.US);

        final LocalDate dtSep20th2023 = LocalDate.of(2023,9,20);
        final String strSep20th2023  = "20 Sep 23";

        String str1 = ddMMMyyyy1.format(dtSep20th2023);
        System.out.println("str1="+str1);
        String str2 = ddMMMyyyy2.format(dtSep20th2023);
        System.out.println("str2="+str2);

        LocalDate dt2 = LocalDate.parse(strSep20th2023, ddMMMyyyy2);
        System.out.println("dt2="+dt2);
        LocalDate dt1 = LocalDate.parse(strSep20th2023, ddMMMyyyy1);
        System.out.println("dt1="+dt1);
    }
}

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

CUSTOMER SUBMITTED WORKAROUND :
Dates in string format have to undergo String replacement .replace("Sept","Sep")  or .replace("Sep","Sept").

FREQUENCY : always