I've ran some tests on JIRA and observed the behavior below:

• Single transition, screen with random fields: JIRA shows the fields and the field "comment", which is not part of the screen.
• Single transition, empty screen: JIRA shows the field "comment".
• Bulk change running a transition, screen with random fields: JIRA shows the fields and the field "comment", which is not part of the screen.
• Bulk change running a transition, empty screen: JIRA shows no field and perform the transition.
  Once the behavior on single transition and bulk change is the same if you have a screen with random fields, I would like that the behavior was the same if you have an empty screen.
  I can't accept the proposed solution because I'll have to change a lot of transitions on my workflows, just to add a field that is already shown. It makes no sense for me.