remove uncomitted flattened many-to-many

Given the following flattened relationship:
Artist<<-->>ArtGroup

The many-to-many join table are named artist_group and has (artist_id and group_id) as a primary key.

I first create 3 ArtGroups. No problem - inserted and everything is ok:
ArtGroup ag1 = (ArtGroup) ctxt.createAndRegisterNewObject(ArtGroup.class);
ag1.setName("ag1");
ArtGroup ag2 = (ArtGroup) ctxt.createAndRegisterNewObject(ArtGroup.class);
ag2.setName("ag2");
ArtGroup ag3 = (ArtGroup) ctxt.createAndRegisterNewObject(ArtGroup.class);
ag3.setName("ag3");
ctxt.commitChanges();

Then I create an Artist, add all the ArtGroups and remove one of them. This is done by a gui as the user are (re-)selecting ArtGroups for the current Artist.

Artist art = (Artist) ctxt.createAndRegisterNewObject(Artist.class);
art.setArtistName("a1");
art.addToGroupArray(ag1);
art.addToGroupArray(ag2);
art.addToGroupArray(ag3);
art.removeFromGroupArray(ag2);
ctxt.commitChanges();

The problem here is the sql that is generated:
INFO QueryLogger: — will run 3 queries.
INFO QueryLogger: — transaction started.
INFO QueryLogger: INSERT INTO ARTIST (ARTIST_ID, ARTIST_NAME, DATE_OF_BIRTH) VALUES (?, ?, ?)
INFO QueryLogger: [bind: 204, 'a1', NULL]
INFO QueryLogger: === updated 1 row.
INFO QueryLogger: INSERT INTO ARTIST_GROUP (ARTIST_ID, GROUP_ID) VALUES (?, ?)
INFO QueryLogger: [bind: 204, 212]
INFO QueryLogger: === updated 1 row.
INFO QueryLogger: [bind: 204, 210]
INFO QueryLogger: === updated 1 row.
INFO QueryLogger: DELETE FROM ARTIST_GROUP WHERE ARTIST_ID = ? AND GROUP_ID = ?
INFO QueryLogger: [bind: NULL, 211]
INFO QueryLogger: === updated 0 rows.
INFO QueryLogger: +++ transaction committed.

PostgreSQL does not like the delete query:
INFO QueryLogger: *** error.
org.postgresql.util.PSQLException: No value specified for parameter 2.
at org.postgresql.core.v3.SimpleParameterList.checkAllParametersSet(SimpleParameterList.java:102)
at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:309)
at org.postgresql.jdbc2.AbstractJdbc2Statement.executeBatch(AbstractJdbc2Statement.java:2485)
at org.objectstyle.cayenne.access.jdbc.BatchAction.runAsBatch(BatchAction.java:164)
at org.objectstyle.cayenne.access.jdbc.BatchAction.performAction(BatchAction.java:114)
at org.objectstyle.cayenne.access.DataNodeQueryAction.runQuery(DataNodeQueryAction.java:95)
at org.objectstyle.cayenne.access.DataNode.performQueries(DataNode.java:309)
at org.objectstyle.cayenne.access.DataDomainFlushAction.runQueries(DataDomainFlushAction.java:255)
at org.objectstyle.cayenne.access.DataDomainFlushAction.flush(DataDomainFlushAction.java:177)
at org.objectstyle.cayenne.access.DataDomain.onSyncFlush(DataDomain.java:828)
at org.objectstyle.cayenne.access.DataDomain$2.transform(DataDomain.java:799)
at org.objectstyle.cayenne.access.DataDomain.runInTransaction(DataDomain.java:854)
at org.objectstyle.cayenne.access.DataDomain.onSync(DataDomain.java:796)
at org.objectstyle.cayenne.access.DataContext.flushToParent(DataContext.java:1261)
at org.objectstyle.cayenne.access.DataContext.commitChanges(DataContext.java:1165)

Derby does not like it either:
INFO QueryLogger: *** error.
ERROR 07000: At least one parameter to the current statement is uninitialized.
at org.apache.derby.iapi.error.StandardException.newException(Unknown Source)
at org.apache.derby.impl.sql.execute.BaseActivation.throwIfMissingParms(Unknown Source)
at org.apache.derby.exe.acf526c18ex010bx3786x9a0dx000000047df04f.execute(Unknown Source)
at org.apache.derby.impl.sql.GenericActivationHolder.execute(Unknown Source)
at org.apache.derby.impl.sql.GenericPreparedStatement.execute(Unknown Source)
at org.apache.derby.impl.jdbc.EmbedStatement.executeStatement(Unknown Source)
at org.apache.derby.impl.jdbc.EmbedPreparedStatement.executeStatement(Unknown Source)
at org.apache.derby.impl.jdbc.EmbedPreparedStatement.executeUpdate(Unknown Source)
at org.objectstyle.cayenne.access.jdbc.BatchAction.runAsIndividualQueries(BatchAction.java:224)
at org.objectstyle.cayenne.access.jdbc.BatchAction.performAction(BatchAction.java:117)
at org.objectstyle.cayenne.access.DataNodeQueryAction.runQuery(DataNodeQueryAction.java:95)
at org.objectstyle.cayenne.access.DataNode.performQueries(DataNode.java:309)
at org.objectstyle.cayenne.access.DataDomainFlushAction.runQueries(DataDomainFlushAction.java:255)
at org.objectstyle.cayenne.access.DataDomainFlushAction.flush(DataDomainFlushAction.java:177)
at org.objectstyle.cayenne.access.DataDomain.onSyncFlush(DataDomain.java:828)
at org.objectstyle.cayenne.access.DataDomain$2.transform(DataDomain.java:799)
at org.objectstyle.cayenne.access.DataDomain.runInTransaction(DataDomain.java:854)
at org.objectstyle.cayenne.access.DataDomain.onSync(DataDomain.java:796)
at org.objectstyle.cayenne.access.DataContext.flushToParent(DataContext.java:1261)
at org.objectstyle.cayenne.access.DataContext.commitChanges(DataContext.java:1165)

Looks like there are two errors here:
1. The delete-query should not have been issued.
2. PostgreSQL and Derby complain about missing parameters in the query. hsqldb does not complain.