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
query = """ INSERT INTO {rel} ({id1}, {id2})
                        (SELECT a, b FROM unnest(%s) AS a, unnest(%s) AS b)
                        EXCEPT (SELECT {id1}, {id2} FROM {rel} WHERE {id1} IN %s)
                    """.format(**parts)
            for sub_ids in cr.split_for_in_conditions(ids):
                cr.execute(query, (records.ids, list(sub_ids), tuple(records.ids)))
        def unlink(ids):
            query = """ DELETE FROM {rel}
                        WHERE {id1} IN %s AND {id2} IN %s
                    """.format(**parts)
            cr.execute(query, (tuple(records.ids), tuple(ids)))
        def unlink_all():