Some people have encountered the problem with previous versions, too, but it doesn't seem to be a global thing.
You can also copy the action out to a text editor and add the variables manually. For example, this is what a Run desktop flow action looks like with no parameters:
@@flowname: '{FlowName}
External.RunFlow FlowId: '{FlowId}'
Where {FlowName} is the name and {FlowId} is the Id of the target desktop flow you want to run.
And this is what it looks like for one that has several input parameters:
@@flowname: '{FlowName}'
External.RunFlow FlowId: '{FlowId}' @CustomerName: $'''CustomerName''' @DocumentVersion: 1
Where 'CustomerName' is an external name of a text type input variable and 'DocumentVersion' is an external name of a numeric type input variable. You can add as many as you need.
When you're done with editing these, you can paste it back to your PAD designer and an action with all those parameters will be added.
