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).


column2=Default, string=Default, **kwargs):
        super(Many2many, self).__init__(
            comodel_name=comodel_name,
            relation=relation,
            column1=column1,
            column2=column2,
            string=string,
            **kwargs
        )
    def _setup_regular_base(self, model):
        super(Many2many, self)._setup_regular_base(model)
        if self.store:
            if not (self.relation and self.column1 and self.column2):
                
                comodel = model.env[self.comodel_name]
                if not self.relation:
                    tables = sorted([model._table, comodel._table])
                    assert tables[0] != tables[1], \
                        "%s: Implicit/canonical naming of many2many relationship " \
                        "table is not possible when source and destination models " \
                        "are the same" % self
                    self.relation = '%s_%s_rel' % tuple(tables)
                if not self.column1:
                    self.column1 = '%s_id' % model._table
                if not self.column2:
                    self.column2 = '%s_id' % comodel._table
            
            check_pg_name(self.relation)
    def _setup_regular_full(self, model):