In Biblio record data entry form, if a field is repeatable, it has on the left of its label a + and - links. With +, you clone the field, with - you delete it. When a field is not repeatable you should have the - (to empty all subfields content) and no +. You have nothing at all.
This happens on 3.00.6 and 3.0.X branch. 3.0.5 didn't have this trouble.
Patch pushed, please test on master