New values get lost during form validation when using select2boxes multiple values widget

Problem/Motivation

When using the Select 2 boxes (Multiple values) widget on a form (in my case, node form) and you add/edit values to the field, if the form server side validation fails then the new values are not saved on the field.

Steps to reproduce

One simple way to reproduce is:
1. Create a dummy Content Type
2. Add a entity reference field to a taxonomy
3. Set the widget to "Select 2 boxes (Multiple values)"
4. Add a list string field or any field that can be displayed as radios, with at least 2 options and make it required
5. Set the widget to radios
6. Create a new node of the dummy Content Type
7. Do not select anything from the radios field (we want the validation to fail)
8. Add at least one value to the select 2 boxes field
9. Save the form and after the validation fails, you can see that the select 2 boxes field is empty, instead of containing the value that we set previously

Proposed resolution

Most probably there's a gap in the code, so debugging would be a good start.

Remaining tasks

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User interface changes

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API changes

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Data model changes

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