Template Tpl_OS contains a low level discovery for filesystems. This LLD contains item and trigger prototypes.
This template is inherrited by Tpl_OS_Unix which adds one or more item/trigger prototypes within the templated LLD (see screenshot), which are unix specific.
When i export/import these templates (either one by one or both at once), the added item/trigger prototypes for the described LLD in Tpl_OS_Unix are not present.
It's present in Zabbix 3.2, probably previous versions as well.