`timedatectl` shows "NTP service :n/a" if timedatex.service is disabled

Issue

timedatectl command always shows "NTP service :n/a", even if chronyd is running and syncing from NTP server.

# timedatectl
               Local time: Fri 2021-03-12 14:37:57 JST
           Universal time: Fri 2021-03-12 05:37:57 UTC
                 RTC time: Fri 2021-03-12 05:37:57
                Time zone: Asia/Tokyo (JST, +0900)
System clock synchronized: yes
              NTP service: n/a    <==
          RTC in local TZ: no
# timedatectl set-ntp true
Failed to set ntp: NTP not supported   <==
# chronyc sources
210 Number of sources = 2
MS Name/IP address         Stratum Poll Reach LastRx Last sample               
===============================================================================
^+ clock01.redhat>     1  10   377   539    +31us[  +31us] +/-  122ms
^* clock02.redhat>     1  10   377   803  +6314ns[+7312ns] +/-  121ms