Why are Word document properties removed when uploading to SP document library?

Question

When we upload a document with custom defined document properties to our SharePoint library, the custom properties are stripped. They are no longer in the document on SharePoint. When they're downloaded from SharePoint they are no longer in the document.

Clarification Update

The Word files we are given have term management properties applied to them within a customer's SP. We amend the Word docs and then upload those docs to our SP. At this point the properties are stripped. We require the properties to stay intact throughout this process. Is there a setting to allow these properties to "pass-through" to our system?

Answer 1

By default, the term management properties in the document will not be moved from the customer’s SharePoint environment to your SharePoint environment.

There are no settings to remain the term management properties in the documents when moving the documents from the customer’s SharePoint environment to your SharePoint environment.

So, you could consider copying the terms in the term store from your customer’s SharePoint environment to your SharePoint environment.