A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
I stumbled on a fix today.
Under system preferences, chose Keyboard, then chose Shortcuts, and then at the bottom under Full Keyboard Access, click the "All controls" radio button.
Previously, the "Text boxes and lists only" radio button was selected. I can recreate the problem by clicking this other radio button, i.e. the fix is fully reversible. I don't think I ever touched either of these buttons before yesterday.
I believe this fix completely resolves for me the issues with selecting text boxes in PowerPoint.
Note that it's very puzzling that a system tabbing option should have anything to do with selecting text boxes in PowerPoint. I expect there is still an underlying bug.