greenforest, it's a bug of Microsoft Word 2003, 2007, 2010, 2013.
1) Turn off auto-save, use manual backuping with versioning for document.
2) Avoid huge number of MS Equation objects in a single document.
3) Strongly avoid using copy-paste for MS Equation objects.
See also,
https://dessci.com/en/support/mathtype/tsn/tsn103.htm