When a Data Source is initially configured and saved, the "Join with other sources", "Join Field" reverts to the first option in the list, not the selected option
When a Data Source is initially configured and saved, the "Join with other sources", "Join Field" reverts to the first option in the list, not the selected option. So you have to save once, return reload / edit the Data Source again and set the "Join Field" again. On the 2nd save, the selected option is retained.
Option as set on the first save.
After the first save, the page is loaded againg and you can see that the option has been reset back to the first option in the list.
Agileware Ref: CIVICRM-1455