NTP synchronized cannot set to yes

CentOS 7.5, 配置过ntpd与某个时间服务器同步后，偶然发现某一个节点的NTP synchronized一直是no:

# timedatectl
      Local time: 二 2019-07-30 09:41:08 CST
  Universal time: 二 2019-07-30 01:41:08 UTC
        RTC time: 二 2019-07-30 01:08:13
       Time zone: Asia/Shanghai (CST, +0800)
     NTP enabled: yes
NTP synchronized: no
 RTC in local TZ: no
      DST active: n/a

停掉ntpd, 执行ntpd -gq重新调整时间后，再启动ntpd:

# systemctl stop ntpd
# ntpd -gq
ntpd: time slew +0.000041s
# systemctl start ntpd

等待一会儿后,NTP synchronized恢复成yes:

# timedatectl
      Local time: 二 2019-07-30 09:44:28 CST
  Universal time: 二 2019-07-30 01:44:28 UTC
        RTC time: 二 2019-07-30 01:44:28
       Time zone: Asia/Shanghai (CST, +0800)
     NTP enabled: yes
NTP synchronized: yes
 RTC in local TZ: no
      DST active: n/a

Ref: https://askubuntu.com/questions/929805/timedatectl-ntp-sync-cannot-set-to-yes