My "activity" model has a stage_id like so:

stage_id = fields.Many2one('activity.stage', string='Stage')

Refering to the "activity.stage" model:

class ActivityStage(models.Model):
    _name = "activity.stage"
     
     name = fields.Char('Stage Name', required=True)
     sequence = fields.Integer(default=1)
     fold = fields.Boolean('Folded by Default')
     active = fields.Boolean('Active', default=True)

The Stage column in Tree View shows the name of the stage_id correctly for several "Activity" records, but when I "Group By..." stage_id, shows 0 records in all stages, also in Kanban View with the default_group_by="stage_id" attribute shows all stage columns but no card records, like I said, only in Tree View is posible to see all existing records.

There is no errors in log file.

Odoo 10.