Bug: WPX9 Ribbon-bar Font-Size cannot be reliably typed by hand

In WPX9, the ribbon font-size box does not work reliably.

Clicking the ribbon-bar Font box, and typing "Century Schoolbook" (any font);
Enter to select or Tab to Font-Size
Tab to font size and type "9"  (9 points);  Enter or Tab to leave the size-box.

Bug: Fontsize returns to previous size (e.g. 11 points); discarding the selected value.

Choosing the Fontsize with the pull-down arrow works correctly.  Unreliable to use keyboard.

This is a new problem with WPX9.  Previous versions did not exhibit this.
Reproduceable in existing and New documents.

Thank you.

* I hate Microsoft Word

Windows 10
WPX9 Version  SourceID: 007003