TwinColSelect weird Behaviour

Hello,
I am using a TwinColumnSelect with a JPAContainer as ContainerDataSource
and setBuffered(true). The Select is bound to a FieldGroup. Everytime the selection made in twincolselct is changed and then defocused by clicking on a button, field etc. a DatabaseException from EclipseLink is printed on the console telling me that a column cannot be null and showing an Insert Query.

[EL Warning]
: 2013-08-14 11:06:38.252--UnitOfWork(1945124820)--Exception [EclipseLink-4002]
 (Eclipse Persistence Services - 2.4.1.v20121003-ad44345): org.eclipse.persistence.exceptions.DatabaseException
Internal Exception: com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Column 'abbreviation' cannot be null
Error Code: 1048
Call: INSERT INTO Table_Name (column1, column2) VALUES (?, ?)
	bind => [2 parameters bound]

Query: InsertObjectQuery(example.generic.datamodel.ExampleEntity[ id=null ]
)

This shouldn’t happen, since defocusing the select should not in any way create a query. This also doesn’t happen if the ItemDataSource is an existing Entity in the Container, it only happens if the fieldgroups ItemDataSource ist set to a newly created EntityItem.

How come?