I'm not sure. Try the bord_RecDescriptor field in Custom_Tables for your custom entities 

The value of bord_RecDescriptor for my custom entity was NULL.  When I changed it to a field with data (surounded by # symbols), it made no difference to the results.  I also refreshed metadata, stopped the QuickFind service, deleted the Quickfind folder and restarted the Quickfind service.  The Quickfind folder is re-created.  In data-config.xml, the query for my custom entity is unchanged, ie it still wants to index the <enty>_Name field and not the field I set in bord_RecDescriptor.

One thing I noticed about the newly created Quickfind\instance\conf files is the file date stamps.  Most of them are dated 31/01/2020 (I'm using CRM 2020 R1) but the data-config.xml is dated 21/03/2021 (ie yesterday).  This suggests to me that the file is being copied from somewhere rather than being generated anew (otherwise it would have today's date) but I haven't managed to figure out where from yet.  If I can find the source file I can try amending the query directly.

The only other thing I can think of is to check the Translations and look up a family called 'SS_ViewFields' and the Code would be the name of your custom entity. 

The only other thing I can think of is to check the Translations and look up a family called 'SS_ViewFields' and the Code would be the name of your custom entity. 

Thanks Matthew.  I've already done that - made no difference.