ADDITIONAL SYSTEM INFORMATION :
Confirmed on Ubuntu x64 Linux and Windows 10.

A DESCRIPTION OF THE PROBLEM :
JDK 8's ZonedTimeTime.parse() correctly parses a date+time with MST correctly but with JDK 11 (and also JDK10.0.2) the value is incorrectly parsed.

REGRESSION : Last worked in version 8u181

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run the attached test case.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Correctly parsed value using Java 8:

  Original value: 2017-01-12T23:00MST
 Converted value: 2017-01-12T23:00-07:00[America/Denver]

ACTUAL -
Incorrectly result using Java 10 and 11ea:

  Original value: 2017-01-12T23:00MST
 Converted value: 2017-01-12T16:00-07:00[America/Denver]

---------- BEGIN SOURCE ----------
import java.time.ZoneOffset;
import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;
import java.time.format.DateTimeFormatterBuilder;
import java.time.temporal.ChronoField;

public class ZonedDateTimeTestCase {

  // comprehensive pattern to handle any type of ISO8601 Date / Date-Time
  private static final String comprehensivePattern = "[yyyyMMdd][yyyy-MM-dd][yyyy-DDD]"
          + "['T'[HHmmss][HHmm][HH:mm:ss][HH:mm][.SSSSSSSSS]"
          + "[.SSSSSS][.SSS][.SS][.S]][OOOO][O][z][XXXXX][XXXX]['['VV']']";

  // formatter used to parse an ISO8601 string using the comprehensive pattern
  // pattern includes default values for time in case time information is missing
  private static final DateTimeFormatter dateFormatter = new DateTimeFormatterBuilder()
          .append(DateTimeFormatter.ofPattern(comprehensivePattern))
          .parseDefaulting(ChronoField.OFFSET_SECONDS, ZoneOffset.UTC.getTotalSeconds())
          .parseDefaulting(ChronoField.HOUR_OF_DAY, 0)
          .parseDefaulting(ChronoField.MINUTE_OF_HOUR, 0)
          .parseDefaulting(ChronoField.SECOND_OF_MINUTE, 0)
          .toFormatter();

  private static final String testDate1 = "2017-01-12T23:00MST";

  public static final void main(String[] args) {
    final ZonedDateTime convertedValue = ZonedDateTime.parse(testDate1, dateFormatter);
    System.out.println(" Original value: " + testDate1);
    System.out.println(" Converted value: " + convertedValue.toString());
  }
}

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

CUSTOMER SUBMITTED WORKAROUND :
None that I'm aware of.

FREQUENCY : always