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.
Just a basic point of clarification first: In order for Tab to demote an item the insertion point must be at the very beginning of the line, whereas the Indent More button will demote regardless of where on the line the insertion point sits. The same is true of Shift+Tab to promote a line. Could this be an explanation?
If not, please indicate your present update level of Office 2016 as well as your version of OS X. Also, specify the exact steps you're taking & whether this occurs in one specific file or whether it occurs in all, including new presentations.
FWIW, I'm running v15.27 on OS X 10.10.5 without seeing that behavior, nor have I seen any similar reports in the Community.