When using the "reuse an existing set" option for a custom field, the default value setting for the original set is ignored, and no option is given to set the default in the second-use field.
Replicated on 2.1 sandbox. See "Group1" and compare "Custom1" and Custom2".
Custom1 has the first option set as default.
Custom2 uses Custom1 option set, but no option flagged as default.