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`` to the cache format; ``value`` may come from an
        assignment, or have the format of methods :meth:`BaseModel.read` or
        :meth:`BaseModel.write`. If the value represents a recordset, it should
        be added for prefetching on ``record``.
        :param bool validate: when True, field-specific validation of ``value``
            will be performed
        """
        return value
    def convert_to_record(self, value, record):
        """ Convert ``value`` from the cache format to the record format.
        If the value represents a recordset, it should share the prefetching of
        ``record``.
        """
        return value
    def convert_to_read(self, value, record, use_name_get=True):
        """ Convert ``value`` from the record format to the format returned by
        method :meth:`BaseModel.read`.
        :param bool use_name_get: when True, the value's display name will be
            computed using :meth:`BaseModel.name_get`, if relevant for the field
        """
        return False if value is None else value
    def convert_to_write(self, value, record):
        """ Convert ``value`` from the record format to the format of method
        :meth:`BaseModel.write`.
        """
        return self.convert_to_read(value, record)
    def convert_to_onchange(self, value, record, fnames=()):