eval() not re-evaluated when objects were modified

I face a problem (bug?): I have a condition using ?eval? which doesn?t seem to be reevaluated when objects are modified.

The class NumberTest wraps a number (field nb) and the index (field index).

The rule gets 2 NumberTest, the first number is greater than the second one ; and the eval condition should test that the first index is less than the second. But the rule is fired even if this condition is not fulfilled (see the check function).