This is really just a warning and can be ignored. What is the issue that you are having? I assume your code isn't expecting this and is causing some unexpected behavior.
We use the Lucene Search library for the indexed search and the base assumption is that when you search you wouldn't expect a significant amount of results. Based on the query provided the search isn't able to provide any unique 'scores' and the results are more generic than one might expect, hence the warning/error.
The only way I see to avoid issues like this is to limit your search to return less results. You could also use the later AFSearch functionality added to the latest release of the PI Web API. This would eliminate any Lucene/Search controller dependency.
C# JsonConvert.DeserializeObject throws an unexpected character exception when this error occurs. Funny thing is that this is a filter that gives fewer results than without the filter. At this time I am filtering the results in C# but would prefer to do it in the query. Thank you for the response.