[brianogilvie]

You cannot set actions, including single actions, to On Hold. On Hold is a property of a project (including a list of single actions). So when you have a single action selected and you use the Inspector to choose On Hold, you're setting the project to On Hold. It's one of the choices in the Project Inspector, not the Action Inspector.

There's been a discussion of how to put single actions on hold. Some people move their On Hold single actions to a blocked context. Others enter a start date in the future (perhaps their next review date for single actions). If it's a feature you would like, send feedback to Omni.