Recently I was dealing with Powerpoint and reopened an old presentation to find that the text in the presentation which had once been transparent was now no longer so.
Turns out there is a trick to this (isn’t there always :-)), which requires you to add a solid line border to the text and change the transparency of this as well. Seems it is something new in one of the recent updates – consensus seems to be this was update 15.20.
I right-clicked on the textbook and selected Format Shape… This bought up the format shape options in the right hand panel. Then ensure there is a solid fill selected for both the text fill and the text outline. See the screenshot below for exact settings.