Hi

By default, One2many fields changes won't update on chatter.

To display tracking of the One2many field in chatter, please refer to the code below.

class A(models.Model):
    _name = "A.A"
    _inherit = ['mail.thread', 'mail.activity.mixin']

    name = fields.Char("Name")
    b_lines = fields.One2many('B.B', 'a_id')


class B(models.Model):
    _name = "B.B"
    _inherit = ['mail.thread', 'mail.activity.mixin']

    a_id = fields.Many2one('A.A')
    name = fields.Char("Name",tracking=True) 
    #To Show tracking in chatter Add tracking=True   

    def write(self, vals):
        super().write(vals)
        if set(vals) & set(self._get_tracked_fields(vals)):
            self._track_changes(self.a_id)

    def _track_changes(self, field_to_track):
        if self.message_ids:
            message_id = field_to_track.message_post(
                body=f'{self._description}: {self.display_name}').id
            trackings = self.env['mail.tracking.value'].sudo().search(
                [('mail_message_id', '=', self.message_ids[0].id)])
            for tracking in trackings:
                tracking.copy({'mail_message_id': message_id})

Hope it helps,

KIran K