1. `copy_cache` in `models.py` recursively copies the caches to environment, 
2. which in
```python
def _compute_related(self, records):
        """ Compute the related field ``self`` on ``records``. """
        others = records.sudo() if self.related_sudo else records
        if records.env != others.env:
            copy_cache(records - records.filtered('id'), others.env)
        values = list(others)
        for name in self.related[:-1]:
            values = [first(value[name]) for value in values]
        for record, value in zip(records, values):
            record[self.name] = value[self.related_field.name]
```
3. copies failing cache values that have been produce previously by this line:

`models.py : def _prefetch_field()`
```python

        try:
            # THIS ONE
            result = records.read([f.name for f in fs], load='_classic_write')
        except AccessError:
            # not all records may be accessible, try with only current record
            result = self.read([f.name for f in fs], load='_classic_write')
```
I checked it's exactly the same `FailedValue` object, which is later accessed without intent (not refreshed) by the fact that the buggy environment is copied to sudo (and not refreshed).


atts.write({'datas': value})
                
                for record in (records - records.browse(atts.mapped('res_id'))):
                    atts.create({
                        'name': self.name,
                        'res_model': record._name,
                        'res_field': self.name,
                        'res_id': record.id,
                        'type': 'binary',
                        'datas': value,
                    })
            else:
                atts.unlink()
class Selection(Field):
    """
    :param selection: specifies the possible values for this field.
        It is given as either a list of pairs (``value``, ``string``), or a
        model method, or a method name.
    :param selection_add: provides an extension of the selection in the case
        of an overridden field. It is a list of pairs (``value``, ``string``).
    The attribute ``selection`` is mandatory except in the case of
    :ref:`related fields <field-related>` or :ref:`field extensions
    <field-incremental-definition>`.
    """
    type = 'selection'
    _slots = {
        'selection': None,              
    }
    def __init__(self, selection=Default, string=Default, **kwargs):