That is the behavior of FM.
FM14 has help entry "value lists troubleshooting" but I couldn't found it in FM15 htmlized help.
If the value list is defined to display information from two fields, items will not be duplicated for the field on which the value list is sorted. For example, if the value list displays information from the Company field and the Name field, and if the values are sorted by the Company field, only one person from each company will appear in the value list.
Thank you for your quick reply however there was a misunderstanding RE your reply. The values are 1:1 and unique- so sorting wouldn't collapse or group any of the records, though that's useful to know.
After reading the document you cited I got the idea to re-index field #2 (itemName), which I did by turning indexing off for field #2, saving via exiting Manage Database, then turning it back on. After this procedure the valuelist populated correctly when sorting by field #2.