Fulltext index is randomly dropped; how to persist?

I have encountered this on 3.5 and 4.0

I create a fulltext per Create and configure full-text indexes.
Using :schema, I see the index. I am able to search and get results for a while
After some period of time (Anywhere from minutes to days), the index disappears, and rerunning the same query throws java.lang.IllegalArgumentException: There is no such fulltext schema index

What actions will destroy this index & how can I keep this index around?

bumping this thread :)