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Currently, Lucene-based Jira search works through indexes which are updated when the Issue is edited. This facilitates and speeds up the search.
The plugin SLA field is a calculable custom field that changes its value every millisecond. We get the value of this field only at the moment when it is addressed or displayed / modified. If the Issue has not been accessed or edited for a while, the index value in Lucene will 'remember' only its last state which can be a years ago, which cannot be considered a reliable result in the search.