Sometimes when you use an Action Set to execute and/or group some actions, it appears when part of the main flow.
It happens that when you have actions nested inside others, you'll have the possibility to mark them to run as the workflow owner only from the principal one.
Only actions at the root path of the workflow will have the Run as Workflow Owner option. If you have an action in any branch, this option will not be available.
It is a good fact to know and workaround it, but I think it is showing inconsistency and hopefully can be fixed.