zabbix manual says on flexible intervals : "If multiple flexible intervals overlap, the smallest Interval value is used for the overlapping period"

this would seem to be intended to use more frequent interval.

but if a positive number interval overlaps with interval 0, item is not checked - that is, 0 is just considered "smaller" than the other number.

it should probably be considered "larger" interval instead.

if this is changed, the behaviour should be also clarified in the manual (item and lld rule pages); if it is not changed, it should be documented