Wrong condition evaluation if the condition contains a variable assignment

I have a rule which checks variables of two equal objects in an old and a new version. If I work with the variable daclaration $nd1 then the consequence is NOT executed although it should.

This is the version of the rule in that the consequence is not executed although it should:

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package apm.event.resource;

#list any import classes here.
import java.util.*;
import de.fraport.test.*;

#declare any global variables here
global Collection results;

rule "Test-Del"
salience 900
when
#conditions
TestMainObjectPair( $omo: oldMainObject != null, $nmo: newMainObject )
TestObject( date1 == null, $od2: date2 != null, $os1: strVal1 != null ) from $omo.testObject
TestObject( $nd1: date1 != null || date2 != $od2 || strVal1 != $os1 ) from $nmo.testObject

then
#actions
System.out.println("****** Executing consequence of rule Test-Del nd1="+ $nd1 +" ...");
results.add($nto);
end

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This is the rule version which delivers the expected result (without using $nd1):

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package apm.event.resource;

#list any import classes here.
import java.util.*;
import de.fraport.test.*;

#declare any global variables here
global Collection results;

rule "Test-Del"
salience 900
when
#conditions
TestMainObjectPair( $omo: oldMainObject != null, $nmo: newMainObject )
TestObject( date1 == null, $od2: date2 != null, $os1: strVal1 != null ) from $omo.testObject
$nto: TestObject( date1 != null || date2 != $od2 || strVal1 != $os1 ) from $nmo.testObject

then
#actions
System.out.println("****** Executing consequence of rule Test-Del nd1="+ $nto.getDate1() +" ...");
results.add($nto);
end

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If wished, I have a corresponding JUnit test case for this issue.