Sometimes assignments have more than one possible method of submission. See if the assignment will accept a file upload, rather than a direct submission from word Online. Even though you are using word online, you should be able to save the file you are working on to your desktop as a .doc or .docx If file upload submissions are accepted, you can upload that saved file, and the format should be preserved.
I have had some students struggle with submissions if they navigate away before the upload is complete. Note the last step in both of the instructions listed above is a confirmation that your file is submitted. As a part of that confirmation, you can also download a copy to view exactly what your submission looks like.
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