combo
boundto=F
controlsource=product.objcode
rowsource = objectcodes
rowsourcetype=2
style=2

Off the top of my head: change the RowSourceType to 3-SQL Statement and put a SQL SELECT statement in the RowSource. There are known issues when using combo boxes in grids when the combo box has RowSourceType 2-Alias or 6-Fields.

FWIW, I never set the column's sparse property to .F. when using combo boxes in grids so that may be why I have never seen the problem that you are experiencing.