Please note that using an external format (DOCX in this case) as the primary working format is very unwise. The support for any external format will always be incomplete. The suggested workflow is using native format with any application (for this case, it would be ODT); and only export to external formats when you need to send the document to someone who can’t open ODT.
Also note that there is LibreOffice version 7.3.4 now, so possibly a bug that resulted in the problem you had was already fixed there. At least simply saving your restored document locally produced a normal document for me.