https://github.com/kiegroup/drools/pull/4055#issuecomment-986571560

This rule is compiled with non-exec-model but throws a compilation failure with exec-model. Note that age is a primitive int.

rule R when
    Person(age != null)
then
end

non-exec-model has to throw a compilation failure if we want the same behaviour. But it's not a major issue. It might break user's (wrong) rule backward compatibility.