It is currently not possible to set field value on a sublist field that is hidden on the form. The field must be exposed in order to set a value. This limitation is tracked under Enhancement 147496.
As a workaround, instead of disabling the Show option for hiding the field, clear its Label instead.
- Navigate to Customization > Forms > Transaction Forms
- Transaction Form: Click Edit/Customize
- Click Sublist Fields
- Label: Remove Value
- Show: Enter Checkmark
- Click Save