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).


odoo/fields.py
""" Convert ``value`` from the record format to the format returned by
        method :meth:`BaseModel.onchange`.
        :param fnames: an optional collection of field names to convert
            (for relational fields only)
        """
        return self.convert_to_read(value, record)
    def convert_to_export(self, value, record):
        """ Convert ``value`` from the record format to the export format. """
        if not value:
            return ''
        return value if record._context.get('export_raw_data') else ustr(value)
    def convert_to_display_name(self, value, record):
        """ Convert ``value`` from the record format to a suitable display name. """
        return ustr(value)
    
    #
    
    #
    def check_schema(self, model):
        """ Make sure the database contains everything for this field. Return
        ``True`` if the schema was altered and the field must be recomputed.
        """
        pass
    def read(self, records):
        """ Read the value of ``self`` on ``records``, and store it in cache. """
        return NotImplementedError("Method read() undefined on %s" % self)
    def write(self, records, value):