The /usr/lib/systemd/system/mdcheck_start.service file states the following:

[Service]
Type=oneshot
Environment="MDADM_CHECK_DURATION=6 hours"

"oneshot" implies that the service will remain in "activating" state for the
entire duration (long, up to the stated 6 hours.

It would be better to use Type=simple, which does not keep the unit in
"activating" mode. This is an important difference in semantics, because
monitoring tools (e.g. prometheus) tend to alert if a unit is stuck in
"activating" for more than (typically) 5 minutes (which usually makes
sense, but not here).

The only side effect that I could find is that systemd will not wait for
follow-up units to start, but since this is a timer-triggered unit,
I would not expect anyone else to depend on this service.

Verified by adding an override file.