Renders stll disappearing

Additional detail on this: primarily (though not exclusively), this is the case anywhere there are captions.   If it has a caption, the renders are absolutely guaranteed to disappear completely within an hour or two.   Other sections of a timeline may retain their renders for longer.   As far as I can tell, the renders are not disappearing from the filesystem - I think they still exist there, based on the fact that I don't think I see the number of files in the Video Preview folder decreasing.   It seems that Premiere simply 'loses track' of them.   A side effect of this is that the drive fills up unnecessarily as one re-renders, as the original renders still seem to exist, meaning that the only 'efficient' way to deal with the disappearing renders is to delete all renders and re-render.