Default Position value doesn't take into account superclasses

Description of problem:
When creating data objects in the Data Modeler, the Position parameter/annotation is mandatory, but the defaults are not handled very well.
Parent data object has a field (of any type) with the default value for Position. If you create a data object that extends this class and add a field while, again, leaving the default Position value, both fields will have Position 0.
If you validate these data object, both seem just fine, no errors are returned.
If you try to to validate a rule (using the second data object) though, an Error pop-up is displayed (see screenshot) with no message. Server log and Problems panel contain messages like this: "Duplicated position 0 for field 'f1' on class org.C2".

Version-Release number of selected component (if applicable):
6.1 ER2

Steps to Reproduce:
1. Create a data object C1 with field f1, leave the default Position value. Save.
2. Create a data object C2, set the superclass to C1, add field f2, leave the default Position value. Save.
3. Validate both data objects.
4. Create a guided rule, add C2 to 'when' section.
5. Validate the guided rule.

Actual results:
Step 3 - no errors.
Step 5 - Error pop-up with no message, validation fails.

Expected results:
A:
Step 3 - Error pop-up warning about duplicate position, validation fails.
Step 5 - Error pop-up warning about duplicate position, validation fails.
B:
Step 3 - no errors - field f2 in C2 is created with a different default value.
Step 5 - no errors.