systemd timer unit behaves differently in between RHEL7 and RHEL8.

Issue

• systemd timer unit behaves differently in between RHEL7 and RHEL8 even if we use same unit file.

• In this example, the timer unit is configured to run shell script via service unit files every minute. The shell script sleeps 90 seconds and does logger commands.

• In RHEL7(systemd-208-20.el7), it looks like the script starts every minute 00 seconds and sleeps for 90 seconds, so it appears that the next execution is skipped.
  In the below example, executions of Jul 5 02:28:00 and Jul 5 02:30:00 seem to be skipped.

  Jul  5 02:27:00 rhel7u1 systemd: Starting timertest...
  Jul  5 02:27:00 rhel7u1 logger[19354]: timertest start
  Jul  5 02:28:30 rhel7u1 logger[19356]: timertest end
  Jul  5 02:28:30 rhel7u1 systemd: Started timertest.
  Jul  5 02:29:00 rhel7u1 systemd: Starting timertest...
  Jul  5 02:29:00 rhel7u1 logger[19362]: timertest start
  Jul  5 02:30:30 rhel7u1 logger[19363]: timertest end
  Jul  5 02:30:30 rhel7u1 systemd: Started timertest.
  Jul  5 02:31:00 rhel7u1 systemd: Starting timertest...
  Jul  5 02:31:00 rhel7u1 logger[19369]: timertest start
  Jul  5 02:32:30 rhel7u1 logger[19370]: timertest end
  

• But in RHEL8(systemd-239-29.el8), it looks like the next execution started immediately after the finish of the previous execution.
  In the below example, the execution finished at Jul 5 03:14:40 and then the next execution started at Jul 5 03:14:41.

  Jul  5 03:13:10 rhel8u1 systemd[1]: Starting timertest...
  Jul  5 03:13:10 rhel8u1 root[5218]: timertest start
  Jul  5 03:14:40 rhel8u1 root[5257]: timertest end          <<<===
  Jul  5 03:14:41 rhel8u1 systemd[1]: Started timertest.     <<<===
  Jul  5 03:14:41 rhel8u1 systemd[1]: Starting timertest...  <<<===
  Jul  5 03:14:41 rhel8u1 root[5264]: timertest start        <<<===
  Jul  5 03:16:11 rhel8u1 root[5302]: timertest end
  Jul  5 03:16:11 rhel8u1 systemd[1]: Started timertest.
  Jul  5 03:16:11 rhel8u1 systemd[1]: Starting timertest...
  Jul  5 03:16:11 rhel8u1 root[5309]: timertest start
  

• Is this behaviour expected?