When you retrieve the accounts list, there is no paging and so, we don't get the full list when there are more than 10 accounts.

I fixed it but I've no clue how to make a pull_request. If somebody wants to do it, it would be a good idea (I'm in 14.0 though...):

def _update_ponto_accounts(self, method='add'):

    url = '/accounts'
    paging_forward = True
    accounts = []
    while url:
        resp_json = self._ponto_fetch('GET', url, {}, {})
        # 'prev' page contains newer transactions, 'next' page contains older ones.
        # we read from last known transaction to newer ones when we know such a transaction
        # else we read from the newest transaction back to our date limit
        url = resp_json.get('links', {}).get('next' if paging_forward else 'prev', False)
        data_lines = resp_json.get('data', [])
        accounts += data_lines

    res = {'added': self.env['account.online.journal']}
    # When we are trying to add a new institution, add all existing account_online_journal that are not currently linked
    # The reason is that a user can first synchronize for journal A and receive 3 bank accounts, he only link one bank account
    # and does nothing with the 2 others, then later he create a new journal and synchronize again with the same ponto token
    # because he would like to link one of the two remaining account to his journal. Since the accounts were already fetched in
    # Odoo, we have to show it to him so that he can link them.
    if method == 'add':
        res['added'] = self.account_online_journal_ids.filtered(lambda j: len(j.journal_ids) == 0)
    for account in accounts:
        # Fetch accounts
        vals = {
            'balance': account.get('attributes', {}).get('currentBalance', 0)
        }
        account_search = self.env['account.online.journal'].search([('account_online_provider_id', '=', self.id), ('online_identifier', '=', account.get('id'))], limit=1)
        if len(account_search) == 0:
            # Since we just create account, set last sync to 15 days in the past to retrieve transaction from latest 15 days
            last_sync = self.last_refresh - datetime.timedelta(days=15)
            journal = False
            journal_id = self.env['account.journal'].search([('type', '=', 'bank'), ('bank_acc_number', '=', account.get('attributes', {}).get('reference', {}))])
            if journal_id:
                journal = [(6, 0, journal_id.ids)]
            vals.update({
                'name': account.get('attributes', {}).get('description', False) or _('Account'),
                'online_identifier': account.get('id'),
                'account_online_provider_id': self.id,
                'account_number': account.get('attributes', {}).get('reference', {}),
                'journal_ids': journal,
                'last_sync': last_sync,
            })
            acc = self.env['account.online.journal'].create(vals)
            res['added'] += acc
    self.write({'status': 'SUCCESS', 'action_required': False})
    res.update({'status': 'SUCCESS',
        'message': '',
        'method': method,
        'number_added': len(res['added']),
        'journal_id': self.env.context.get('journal_id', False)})
    return self.show_result(res)

I also added a functionality to retrieve the existing journals but it doesn't work with the wizard yet => You need to cancel and to synchronize afterwards.