AEM 6.0 query issue on cq:tags

Hi All,

We started seeing a weird issue about query optimization a few months ago. When we issue a query as:

/jcr:root/content/site//element(*,cq:PageContent)[@sling:resourceType='site/components/page/news-detail' and (@cq:tags = 'department:drama')] order by @date descending

We will see a warning message about traversing too many nodes, 

org.apache.jackrabbit.oak.plugins.index.property.strategy.ContentMirrorStoreStrategy Traversed 60000 nodes (212558 index entries) using index cq:tags with filter Filter

There is a node "cqTags" under oak:index, as

The weird thing is that if we modify the query a bit, /jcr:root/content/site//element(*,cq:PageContent)[@sling:resourceType='site/components/page/news-detail' and (@cq:tags = 'articles:drama')] order by @date descending. Changing the tag value from "department:drama" to "articles:drama" will not issue any warning, given that both tags are valid in the /etc/tags. Also, the issue only takes place in the publisher, not the author. 

The "department:drama" tag property is as:

Any idea where I should troubleshoot this?

Thanks!

-k