This seems to be a bug that has carried through from 1.31 to 1.33b that the scroll text feature for subtitles doesn't seem to work.
If you create a Play Speech action with scroll text on the text plays one character, waits the appropriate time and then disappears.
Turning off scroll text option and the whole sentence appears as normal.
Comments
It was because the subtitles option is set to auto size. Needed to make the box set to manual and make it bigger to scroll into.