The Spatial PrefixTreeStrategy will index the center-point of a non-point shape it is given to index, in addition to the shape itself of course. The rationale was that this point could be picked up by PointPrefixTreeFieldCacheProvider for distance/sorting. However this approach is buggy since the distinction of grid cells between the center point and the shape itself is lost when the shape gets indexed down to max-levels precision – each grid cell therein appears to be another point that needs to be brought into memory. It's also possible that the shape is a LineString or some other non-trivial shape in which its center point isn't actually in the shape.
Even if you knew this problem would never happen, I think you're better off indexing center points into another spatial field if you want them. Perhaps arguably this strategy could do that internally? Wether or not that ends up happening, I just want to remove the problematic behavior now.