Unassigning contact subtype for a custom group causes data loss

I'm having a trouble unassigning a contact subtype for a custom group of fields...

I've contacts with multiple contact subtypes e.g. student and parent. If the custom group is assigned to both, the student and parent, and I unassign the custom group to the student, data is removed also from parent contact subtypes.

CiviCRM launches this warning before continue the process:

Warning: You have chosen to remove one or more subtypes. This will cause any custom data records associated with those subtypes to be removed.

But I think that if a contact with the subtype unassigned have an other subtype keep making the custom group, data should not be deleted.