Hi,

can you try the code below

class TimesheetEmployee(models.Model):
    _inherit = 'account.analytic.line'

    @api.onchange('employee_id')
    @api.depends('employee_id')
    def _onchange_employee_id(self):
        for analytic in self:
            return {'domain':
                {
            'employee_id': [('user_id', '=', analytic.task_id.user_id.id)]}
                }