Polygon Neighbors tool produces incorrect results

Question

BACKGROUND:

At 10.8.1, I have used the Polygon Neighbors tool with a countywide parcel shapefile (approximately 47,000 polygons). The tool runs to completion (taking < 1 minute) without reporting any errors. I used the shapefile's PARCEL_NO field as the "Report by Field", and I unchecked "Include both sides..." option. All other settings were left at their default.

THE PROBLEM:

When I manually check the results, I find that Polygon Neighbors has failed to identify many of the neighboring polygons. For example, one of the ~47k polygons has 15 neighbors, but the tool only found 4 neighbors.

In several cases, none of the surrounding polygon neighbors were identified.

Out of approximately 20 manual checks, only one parcel's neighbors were correctly identified, and that was a simple situation with only two neighbors.

ATTEMPTED SOLUTIONS:

1. Ran the Geometry Check tool on the parcel shapefile. It reports ok.
2. Ran Polygon Neighbors from ArcCatalog instead of ArcMap. The same errors resulted.
3. Selected a subset (169) of the ~47k polygons and Data > Export them to a new shapefile. This subset included the polygon with the 15 neighbors mentioned above. In this case, Polygon Neighbors correctly(!) identified all neighbors.
4. Used QGIS to perform the same polygon neighbor concept with the ~47k polygon shapefile, using the methods described in https://spatialthoughts.com/2019/05/23/neighbor-polygons-aggregate-qgis/. This method correctly identified all neighbors. However, it took about 20 hours to complete, so I'd like to find a faster option.

QUESTION:

I've run out of ideas. Why does Polygon Neighbors fail to identify surrounding polygons in the ~47k polygon shapefile, yet correctly identify them in a much smaller shapefile subset?

Answer 1

This is what tool does if you uncheck "Include both sides" option:

So if you'll decide to compute number of neighbors by counting frequency of src_OBJECTID, you'll end up with very few (first record only?) correct answers:

Tool essentially creates edges of a graph, with polygons being graph nodes. Uncheck option if you are dealing with undirected graph. It helps to solve multiple tasks, e.g. graph coloring, or equal totals grouping.

"Both sides" option is useful when dealing with directional graph, where edge weight in one direction might be different to one in opposite direction, e.g. during pipe route design. It also useful when you want quickly compute number of grumpy neighbors: