When dashboard filter is set to not show low severity issues, less than 20 issues may be reported.
It seems that Zabbix logic is to take 20 latest issues and filter out those not matching the filter.
It is then possible that "Last 20 issues" is empty even with high severity issues still active if more than 20 low severity issues are more recent.

Limit to 20 elements should be applied after filtering out low severity issues, not before.