(1) The description uses the term "text area", but that is not defined and can easily be confused with draw:text-areas.
(2) The text does not clearly state, what is used as reference for alignment. "inside a shape" is too vague and does not consider, that custom shapes have a defined area for text.
(3) It is not clear, what "justify" means, especially in what aspect it differs from "center".
(4) Handling of overflowing text is not specified.

I have uploaded a document to
https://beta.opendocumentformat.org/rendercompare/upload/266/87/192/1
The document examines primitive ellipse, custom shape ellipse, text box, word wrap and padding.
The text in the shape consists of four paragraphs. The first paragraph is left aligned, the second is centered, the third is right aligned.

The comments are in regard to LibreOffice. You will notice, that PowerPoint renders it different. Look especially at the right aligned paragraph.

I have no proposal yet, because the desired behavior is not clear.