Shapes in Writer are provided by LibreOffice’s drawing layer — they are independent from the normal Writer paragraphs. Given that the drawing layer does not support tracking changes, just Writer’s "native" paragraphs, fully featured tracked changes in real shape text would be quite some work. In case of ODF, the markup describes tracked changes in a way, so that in case the reader does not support tracking changes, it can at least read the normal and inserted text, i.e. the current version.
This is exactly what I implemented in the DOCX import filter now:
Previously we just ignored both inserted and deleted text, so if you had content which was all either deleted or inserted, you ended up having no shape text at all (can be tested using e.g. this test document):
To be fair, the reference layout looks like this:
I still hope to fix that as well one day, but the above fix is something we’ll already provide in 4.3. :-)