validate() fails unit-tests because of a failed assertion in StateEvaluator.evaluate()

[yamaking2]

Describe the bug
When using unified-planning/engines/TimeTriggeredPlanValidator in unit-tests, a call to unified_planning/model/walkers/state_evaluator.py::StateEvaluator.evaluate() fails the assertion assert r.is_constant(). The following print shows {expression} {r} {r.is_constant()}:

Both expression and r are of type OperatorKind.EQUALS, and this is why the assertion fails. If we mock the function evaluate with:

the assertion now succeeds. This leads us to believe that r is not being properly evaluated, and we're not sure why.

Expected behavior
We expect there to be no errors raised in our valid PDDL plans.

Additional context
The same PDDL used to work fine, but now it doesn't. We are not sure when did this assertion start failing.