Today I added a shadow to the font since the video background I added was very light in colour.
Afterwards, the words were all there, but positioned at the bottom of the slide.
I tried several things, including removing all the text and substituting some test text – still positioned at the bottom.
Then when checking the theme settings again, the words moved to the centred-vertically position.
Just a glitch? I could find no setting to set the vertical position.