ADDITIONAL SYSTEM INFORMATION :
macOs High Sierra version 10.13.5

java 11-ea 2018-09-25
Java(TM) SE Runtime Environment 18.9 (build 11-ea+21)
Java HotSpot(TM) 64-Bit Server VM 18.9 (build 11-ea+21, mixed mode)

A DESCRIPTION OF THE PROBLEM :
DateTimeFormatter.format​(TemporalAccessor temporal) throws a DateTimeParseException on encountering a 'Z' depending on the locale the DateTimeFormatter is constructed with. In this case the exception is thrown if Locale.CANADA is used but not if Locale.US is used.

This appears to be a regression as I cannot reproduce the error in Java 8, 9, or 10

REGRESSION : Last worked in version 11

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run the test case below (javac DateFormatBug.java; java -cp . DateFormatBug) and in Java 11 it throws an exception:

Exception in thread "main" java.time.format.DateTimeParseException: Text '1970.01.01 at 00:00:00 Z' could not be parsed, unparsed text found at index 23
at java.base/java.time.format.DateTimeFormatter.parseResolved0(DateTimeFormatter.java:2049)
at java.base/java.time.format.DateTimeFormatter.parse(DateTimeFormatter.java:1874)
at DateFormatBug.main(DateFormatBug.java:18)


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The date time string should be parsed without error. If the locale Locale.US is used the exception isn't thrown. Java 10 does not throw the exception for any locale I tested.
ACTUAL -
A DateTimeParseException is thrown.

Exception in thread "main" java.time.format.DateTimeParseException: Text '1970.01.01 at 00:00:00 Z' could not be parsed, unparsed text found at index 23
at java.base/java.time.format.DateTimeFormatter.parseResolved0(DateTimeFormatter.java:2049)
at java.base/java.time.format.DateTimeFormatter.parse(DateTimeFormatter.java:1874)
at DateFormatBug.main(DateFormatBug.java:18)


---------- BEGIN SOURCE ----------
import java.time.Instant;
import java.time.ZoneOffset;
import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;
import java.time.format.DateTimeFormatterBuilder;
import java.time.temporal.TemporalAccessor;
import java.util.Locale;

public class DateFormatBug {

    public static void main(String [] args) {
        DateTimeFormatter formatter = new DateTimeFormatterBuilder()
                .parseLenient()
                .appendPattern("yyyy.MM.dd 'at' HH:mm:ss z")
                .toFormatter(Locale.CANADA);

        String datetime = formatter.format(ZonedDateTime.ofInstant(Instant.ofEpochSecond(0), ZoneOffset.UTC));
        TemporalAccessor parsed = formatter.parse(datetime);
    }
}
---------- END SOURCE ----------

FREQUENCY : always