I’ve run into this situation recently, and I would think this is a bug.

Let’s say you are a System Customizer, and you have a read-only form and an edit form for an entity, say Account. Now you try to create a Workflow for Accounts, and when you get to add an Update Record step, and click on the Set Properties button, you are presented with the read-only form. Annoying  enough.

The reason for this behavior is because Dynamics CRM remembers the last form type you have used. If the last time you accessed an Account you’ve used the read-only form, you are presented in the Workflow with that same form.

To avoid that, go into an Account, and change the form to the edit form. Save and go back to your Workflow. Now you should be getting the proper edit form.

Hopefully we see a fix for this soon.