Instance being added when form is saved with extended rights

I created a LiveCycle form containing two tables each with a single header row. The first row in Table#1 contains two buttons:

A "Plus" Button => adding a new line to Table #1 and simultaneously a new line to Table #2

Code (JavaScript):

this.parent.parent.instanceManager.addInstance(1);
xfa.form.TEST.Table2.Tabelle2.Zeile1.instanceManager.addInstance(this.parent.parent.instanceIndex);

A "Minus" Button => removing the selected line from Table #1 and simultaneously from Table #2:

Code (JavaScript):

xfa.form.TEST.Table2.Tabelle2.Zeile1.instanceManager.removeInstance(this.parent.parent.instanceIndex);
this.parent.parent.instanceManager.removeInstance(this.parent.parent.instanceIndex);

All this works fine as long as I do not save the file with extended form rights enabled. After having saved the form with extended rights and opening the file with Adobe Reader 9.1, there are two instances in Table #1 (Instance 0 and 1) and Instance 1 can not be deleted anymore. In Table #2 however, there is only Instance 0 (which is ok).

How can I fix this problem?