The 'Del' key is removing the registers of a o2m instead of deleting.
As it's described in this discussion: https://discuss.tryton.org/t/new-behavior-when-using-the-supr-key-on-a-one2many/1836/7, the remove variable should be forced to false when add_remove attribute is not set (http://hg.tryton.org/tryton/file/default/tryton/gui/window/view_form/view/form_gtk/one2many.py#l196).