After 1000 entries/lines it stopped working...... which is fine, as you have other options to handle it, but the nasty thing was, when accessing a form through "caller" where the reference normally is set automatically, this ALSO fails to work.... which resulted in orphans in the database.
Lex van Rooy
This is a known issue, see for example this thread. "Worse, when an existing reference has a value that is in de non-visible range of the dropdown the value is shown as blank. After commit the value is gone and not-null constraint are triggered (value is not visible/reselectable)."