PartOf= does not propagate restart to units in activating state

systemd version the issue has been seen with
242-7

Used distribution
Debian Unstable

Expected behaviour you didn't see
When calling systemctl restart on a unit, units with a PartOf= dependency should be restarted, including those in activating (start) state.

Unexpected behaviour you saw
Units with a PartOf= dependency that are in activating (start) state are not restarted.

Steps to reproduce the problem

Create a dummy test-a.target unit.

Create a test-b.service unit with the following:

[Unit]
PartOf=test-a.target
[Service]
Type=oneshot
ExecStart=/bin/sleep 10

Run:

systemctl start --no-block test-b.service
sleep 3
# test-b.service is in activating (start) state
systemctl restart test-a.target

You will notice the third command does not restart test-b.service, despite the PartOf= dependency.

Additional information

This behaviour is surprising because systemctl stop test-a.target does propagate correctly, and systemctl restart test-b.service does work, even if test-b.service is in activating state.

Workaround

One workaround is systemctl stop test-a.target; systemctl start test-a.target, but that only works if test-a.target has a dependency on test-b.service.

Reproduce the behavior with the test-a.target and test-b.service units, then run the listed systemctl commands while test-b.service is activating. Trace how PartOf= restart propagation handles that state; done means restarting test-a.target also restarts the activating test-b.service, while existing stop behavior remains intact.