Hi Alejandro,
This looks like a bug and overlook to me but as Tim pointed out simply changing it would be backward incompatible at this point.
If we were to fix this issue we would need to deprecate the actual behavior first. The deprecation path would involve raising a warning when the actual related_query_name is referenced (e.g. through Queryset.filter(), annotate(), ...) if default_related_name was defined and then switch to the new behavior once the deprecation period end.
If you are really motivated into fixing this please file
a new ticket to track your effort.
Simon