Newer versions of your Office applications (Word, Excel, etc.) store the properties from your SharePoint libraries within your Office documents.
You ignore a metadata field from the options in a Copy Content migration, and the field still has a value for some items at the destination.
The ShareGate migration tool ignores the metadata values as established in your copy options, but SharePoint detects these values in your recent Office documents and applies them in your columns automatically.
There is no solution for this issue.