I have created an External Object via Files Connect and linked up a custom field to a custom metadata column in SharePoint.
The field displays correctly, when I change data in SharePoint it gets updated pretty much instantly in SalesForce, etc. However, when I try and query this field using SOQL, it always returns null (seems to do this for every custom field on the External Object). When I filter the request to check for nulls (i.e. SELECT Name__c, CustomField__c FROM ExternalObject__x WHERE CustomField__c != NULL), it seems to ignore the WHERE clause and still returns all objects (even ones whose fields are empty in SharePoint), and returns CustomField__c as null (it still returns other fields such as Author__c and Name__c correctly, just the custom field is null).
I have tried following the documentation (here: https://help.salesforce.com/articleView?id=admin_files_connect_sp_custom_props.htm&type=5), but when I try to use ManagedProperty Column names and Alias's, I just get apex errors back saying those fields don't exist on the External Object.
Any ideas on what I'm doing wrong?