QGIS expression: when using overlay_nearest sometimes a feature considers itself its nearest neighbour

Question

When using overlay_nearest function in the QGIS expression builder, sometimes a feature considers itself as the nearest geometry, and other times not.

To demonstrate, I made a label expression:

CASE
    WHEN
        letter='A'
    THEN
        'fid: ' || "fid" || 
        '\nletter: ' || "letter" || 
        '\n\nnearest fid\'s: ' || array_to_string(
                                      overlay_nearest(layer:=@layer, 
                                          expression:=fid, 
                                          filter:=letter='A', 
                                          limit:=2
                                      )
                                  )
    ELSE
        NULL
END

I set the feature limit at 2 to show the 2 nearest fids for each feature to show that the true nearest feature is being found second.

The points outlined in red consider themselves their nearest neighbour, the others do not. It happens also with a projected coordinate system, and with various permutations of the WHEN clause.

Can anyone explain why this is?

Example dataset (CRS is WGS84) -- GeoPackage download link (with label expression pre-loaded)

WKT	fid	letter
POINT (27.7981358844261 -24.2696491141906)	1	A
POINT (27.7988424066944 -24.2678611535372)	2	A
POINT (27.7989643301811 -24.2688320385901)	3	B
POINT (27.8007927306901 -24.2685965478542)	4	B
POINT (27.7979157991226 -24.2678537822582)	5	B
POINT (27.7995823651664 -24.2670624651733)	6	A
POINT (27.7999025692956 -24.2692146208195)	7	B
POINT (27.8002376476489 -24.2696982965109)	9	A
POINT (27.7984109576877 -24.2694592018388)	10	A
POINT (27.8001260102486 -24.2667596896059)	11	B

Answer 1

The overlay_nearest function and others, respond badly if used with the same layer.

Here is a bug report: https://github.com/qgis/QGIS/issues/47201

Answer 2

So far I noticed that the overlay_nearest will result in an array, where the same feature will always be at the 0-index position.

Therefore, like in this thread Drawing 10 nearest lines from each point connecting to the centroid of the 5 points nearest to each point with QGIS expression, I am simply suggesting to delete it by its index i.e. 0.

However, in such a case, the number in limit should be incremented in one more feature.

array_remove_at(
    overlay_nearest(
        layer:=@layer,
        expression:=fid, 
        filter:=letter='A', 
        limit:=3 -- notice this change
        )
    0)