I would suspect that the drop you are seeing has as much or more to do with the keyword dropping out of the Autocomplete list rather than the Instant Results feature which seems to draw from the Autocomplete dataset.
If your targeted keyword doesn’t meet a certain threshold of search volume it will not appear in the Autocomplete list and as a result, it will not appear in the Instant Results either. It may have had enough volume last month but with a new dataset, it may have not quite reached that threshold of search volume.
There are other situations that could cause your keyword to not appear in Autocomplete, but the most likely is that other phrases had more volume in the latest data export.
