I tried reproducing that configuration in a new document library but there everything kept working, so the issue had to be with the existing library.
Further analysis, comparison and reflection showed that the problematic library was missing some Taxonomy-related Event Receivers.
I tried other fields, and they got written to the item just fine.
After some trial-and-error and consulting the MSDN I found a solution.
It also provides some related Office Information, including VSTO and VSTA and other office application development tips.
In Share Point, we can create list item event handlers (or receivers).
It specializes in Microsoft Share Point technologies, including web parts, development, configuration, customization, and best practices for the use of Microsoft Share Point Server and Windows Share Point Services.Today I was troubleshooting a customer farm where Managed Metadata would remain empty in Share Point, even though it was filled in correctly in the document’s Document Information Panel.Digging through the internal XML structures of the DOCX and also the Content Type Schema and Field XML, I couldn’t find a reasonable explanation.Scenario Overview: Custom event handler for a Share Point List with lots of columns.The Item Updating logic needs to compare various dates and number fields to determine if the data could be submitted.