This is unintuitive, but merely running a saved search from the user interface does NOT create an entry in that saved search's "history". Only the scheduler can populate the official history.
So the given saved search, although set to run on a schedule, has not actually run its first scheduled job yet.
One quick and dirty way to workaround this, at least while developing a view or a dashboard, is to set the schedule to 'every minute', wait a minute, then set it back to whatever your proper schedule was... Then you'll have a placeholder job to hold you over until the real job runs at 2AM or whenever.
'backfill' is term that I've only seen used in regards to Splunk's summary indexing feature, which is different from search-scheduling.
You need to set useHistory to Auto (the default), or just remove it. Setting it to true requires that a previous instance of the job be run, saved, in the scope of the UI module (i.e., in the same app or exported to system), and readable by the user viewing the UI module. Is all of this true?
I've checked those criteria and they are in place.
Swapped the useHistory parameter over to auto results in the dropdown field being blank. I suppose splunk is not detecting the backfill job for which I saved the results and setting the dropdown to empty.
Thanks heaps for the response though.
At this stage I am considering placing all of data required by the report in a summary index. This will mean I can quickly populate the dropdown using an inline search string.