When Japanese is the default editing language, PowerPoint has an UI option to apply Japanese-language text rotation, which rotates letters and digits differently . However, this property seems missing from the object model and an add-in therefore cannot know its state. Therefore, the add-in is bound to handle such text incorrectly.

Apply the rotation setting from scratch:
1) PowerPoint > File > Options > Language > Choose Editing Languages > Japanese (set as default) > OK
2) Restart PowerPoint
3) Create new empty file
4) Insert textbox
5) In textbox, write test123
6) Select textbox
7) PowerPoint > Home > Paragraph > Text Direction > Vertical
8) Save file

RESULT:
The text is rotated such that "test" is rotated 90 degrees clockwise, while the digits "123" are not rotated.

9) Rename the resulting file to *.zip and unzip it. Navigate to ppt\slides and open slide1.xml. The associated XML property seems to be "kumimoji": <a:endParaRPr lang="ja-JP" altLang="en-US" kumimoji="1"/>

Compare the definition in the ECMA Office OpenXML - Fundamentals and Markup Language Reference:
"Kumimoji specifies whether the numbers contained within vertical text continue vertically with the text or whether they are to be displayed horizontally while the surrounding characters continue in a vertical fashion." (e.g., section 21.1.2.3.2 defRPr (Default Text Run Properties)).

It should be possible for an add-in to check if a text run's properties contain this setting and to change it.