Here are the steps to reproduce this problem:
1. Insert any image in a Writer document
2. Right-click on the image to pop-up the context menu and choose "Description..."
3. Put something in the box for Description
(Notice that by default, the image is anchored *to paragraph*)
4. Change the Anchor to something else, eg "To character"
5. Check the Description again and unfortunately, the content is gone!

I've recently re-edited some old documents by changing some anchors.  But I didn't realize that Description contents are removed at the beginning and I've lost quite a lot of descriptions!

Image description field is actually very strange.  Once I noticed the loss of descriptions, I tried to do some "undo" to save back as many as I could.  But I have to do several "Undo" and **Redo**: one or two undo(es) are not enough and the description remains empty.  Sometimes I have to do three or four undo to go back, then a few redo to move forward and somewhere in between, Description field is not empty.