Debugging: Breakpoints are only considered for code with variables

In my installation breakpoints in drl files are only recognized
if there is any variable involved. Here is a small example

1) Works ok, while debugging the application stops at the breakpoint in
System.out.println()

rule "TestRule"
when
eval (1==1)
then
int i = 1;
System.out.println("Breakpoint ok" +i);
end

2) The breakpoint in System.out.println() is ignored. The rule fires
nevertheless....
rule "TestRule"
when
eval (1==1)
then
int i = 1;
System.out.println("Breakpoint ignored");
end

Tested via the Debug > As > Drools