We have discovered a glitch with SharePoint that prevents the usage of a modified column in a content type in document libraries that use that content type.
If you have an existing site column used by your content type in SharePoint, you may wish to update it. In our case, we wanted a lookup column to also return the ID of a record so that we could use that ID in filters within the hosting document libraries.
We navigated to site settings, clicked site columns, and located the lookup column that was associated with our content type. We added a checkbox beside the "Add a column to show each of these additional fields - ID" value. And clicked OK.
When we looked at the content type itself, you cannot see the ID values reflected here. This is normal. The Content Type page does not reflect additional fields for lookup values.
However, we could also not see the ID field in our document libraries that used this content type. Even if we created a new document library, and added the content type, the ID field was still not available.
As a last resort, we DELETED the site column from the content type that had the lookup. We then re-added the same column that was just deleted from.
Now, when I navigate to any document library that uses the lookup column, I can also see the ID value for that lookup column. This can now be used for view and web part filters to restrict document records to those that just match the lookup value.