NumericRangeValidator firing for unrelated fields

On a single form, I have two form fields that use DropDownDataGrids to populate from lookups.

As the ID is an int I am using NumericRangeValidators set to Min > 0 in place of RequiredFieldValidator which does not work with int fields.

However despite being correctly configured (each one on it's own on the form works perfectly), when completing the form, both NumericRangeValidators fire for both dropdowns.

Is this a bug or is special configuration required for this scenario?