Its probably the security constraints. I also noticed that System.owner is empty on some of the results, might be related.
Are there security constraints applicable for the logged-in User/role?
The data flow is for Administrators and Users, as is the screen.
I am logged in as an Administrator.
Thanks Rene and Andrej,
I am using multi-tenant. The blank values are rows for tenants other than the currently logged in user's tenant!
(Andrej, after your comment, I looked properly at the debug data ...)
I have XPath filters defined on each Entity, for each Access Rule, to enforce the mutli-tenancy but the dataflow retrieve actions are ignoring the XPath filters and hence returning rows for all tenants. It seems as though the UI is respecting the filters and so not showing the data - hence the blanks.
I copied the XPath constraint from the Entity access rule to the Retrieve action and it now works as expected.