Does upgrading of catalog scramble Keyword hierarchies?

I have a large keyword hierarchy with several layers of parent levels and subcategory levels.  After catalog upgrade my keyword structure is in disarray, with keywords scattered into various levels parent to their correct placement in the heirarchy.  They aren’t merely duplicated outside their proper place; some keywords have been removed and are missing from their proper place in the hierarchy.  I understand that keywords can appear to be “misplaced” if files are imported from an old folder that was improperly moved outside of Lightroom control.  This seems different.  I may have done something wrong, but I don’t know what it was.