Separate contexts for each job
When working with ARM resources using a run-as account, all concurrently running jobs share the same context. So if one job uses Set-AzureRmContext to switch to a particular subscription, all other jobs will have their context changed to the same subscription.
This makes the scenario of executing jobs against different subscriptions in parallel impossible.
Thanks for opening this feedback. We are looking at ways to enable multiple subscriptions from a single automation account. Please add your scenario to https://github.com/Azure/azure-powershell/issues/1257 also so that the issue can be prioritized.
James Wellington commented
Looking at the 1257 link, it is marked as resolved through various other fixes. Due to my limited knowledge I don't understand how it has fixed it.
Could someone please explain how we can now implement jobs running in parallel on different subscriptions using the same RunAs account and automation account (or multiple runbooks running at the same time in different contexts on the same RunAs account and automation account). I have seen this as an unresolved issue elsewhere too.