I’ve made my own version of the Resource load diagram example
https://docs.dhtmlx.com/gantt/samples/11_resources/04_resource_usage_diagram.html
but the owner doesn’t change when I save the lightbox changes. The description and time period are updated.
So I haven’t got the connection between the lightbox and the gantt correct somehow.
How can I identify what is going on?
Found it ten minutes after posting the question! 
Thanks to the guide: https://docs.dhtmlx.com/gantt/desktop__lightbox_manipulations.html
I noticed that my map_to needed to be “user” and not “owner”.