Steps to reproduce:

1. Start monitoring "many" hosts for issues (>50?)
2. Trigger an alert on all hosts somehow
3. Navigate to overview page, clear all filters.

Result:
Only some triggered alerts are seen.

See screenshots with full problem list, overview missing triggers and overview with filter restricting number of hosts and showing more triggers.
Expected:
Expect to see the alerts for all hosts

Further Information

Adding > 50 hosts to the filtered hostgroup starts to remove a host from the overview page.

Editing file /usr/share/zabbix/include/defines.inc.php
 
// Applied to tables with variable count of columns. Columns above this limit will be truncated.
define('ZBX_MAX_TABLE_COLUMNS', 50);
Increasing that number to > host count seems to bypass the problem.

Similar issues on forum with overview page https://www.zabbix.com/forum/zabbix-troubleshooting-and-problems/403187-alerts-hidden-on-overview-page

and Latest data https://www.zabbix.com/forum/zabbix-troubleshooting-and-problems/403544-latest-data-selection-data-missing