1. The problem is present in both my Win10Pro PCs. Office 2013 in one, 365 in the other.
2. Ribbon tabs return to default order after a close/restart of Word
3. The file Word.officeUI seems to contain specific info for add-in tabs only when they have been moved. After a restart of Word, all info of the add-ins has disappeared from Word.OfficeUI, but the tabs as such remain and are operative.
-Lup