We recently ran into an issue with Metadata columns in SharePoint used by document based content types.
A custom document content type was created with multiple fields for both classification and workflow tracking in the system. Two of these fields were multi line fields to allow comments during the review cycle.
When a document library was associated with this content type, we could correctly update all of the fields using the "Edit Properties" and see them reflected. All except the multi-line fields. You can fill out these fields, click save, and never see the fields updated with the values entered.
Luckily, the fix ended up being pretty simple. Within the DOCUMENT LIBRARY that is hosting the content types (not the content types themselves), change the multi line text fields to single line of text fields and save. Then revert those fields back to multi-line fields, and you can now enter data into those fields and have them save.