In the above macro, I have a "Prompt for User Input" with 4 variables, which show up as 4 fields on the form. Pressing TAB has no effect. In all forms I've every used, TAB will move to the next field, and SHIFT-TAB moves to the prior field.
The fields are not text fields, they are popup menus. Normally the Mac does not tab through them, unless you turn on tab through all controls (see System Preferences ➤ Keyboard ➤ Shortcuts (I don’t know why its in shortcuts).