Dataflows should verify using actual library variables

For this particular case, we recommend using the Variable.Value approach which is now supported. This will make sure that your validations and every single evaluation has to be evaluated against the actual variable library. You can also wrap this around a "try ... otherwise ...." approach (as described here Error handling - Power Query | Microsoft Learn) to make it a bit more robust and provide some other mechanisms to fail in the event that the variable couldn't be loaded.

The Variable.ValueOrDefault has an internal mechanism by design that, on any issues retrieving the variable, it'll use the default value so it wouldn't truly fail. There are other ways in which you could tackle this by also leveraging a conditional mechanism for the Default value and validating other components, but they are not as straightforward as just using the Variable.Value approach.

Hope this helps!