This was already discussed in Sync plan gets stuck in running state if one at least one repo failed
@aruzicka found already the cause: V2::WithSubPlans ignores tasks in stopped-warning · Issue #434 · Dynflow/dynflow · GitHub
But it requires still a fix, so perhaps he can give an update.
There was also Bug #36925: Sync plans in 3.8/4.10 become stuck in running state when any repo fails - Katello - Foreman created for tracking.