There no way to change the default.
The reason why the anchor was changed is saving in .docx whose internal specification has not the same anchors as ODF. Welcome in the world of document compatibility and exchange
Anchoring to page is a bad idea. The object (image, frame, table, …) becomes anchored to this specific page (let’s assume for the explanation it is page 10) and no longer participate in text flow. Now you edit your document. Two things may happen:
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you added material before the image
Text is reorganised and new text will push existing one around the image. Consequently, text will no longer be in sync with the image (if that’s important for your document). Whatever you added, the image is still anchored to page 10.
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you removed material before the image
Text is again reorganised, pulling existing text after the image to a position before it. Imagine now you erased a huge sequence of paragraphs decreasing your content to 5 pages. The image is still anchored to page 10, creating empty pages 6-9. These empty pages are so artificial that you can’t remove them through editing your document. To do that, you must changed the image anchor, which is not obvious for most Writer users!
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