Bug in NearestNeighborGraph

Assuming you are asking for a workaround, you can generate the output you expect with:

v = Style[#,20]& /@ Alphabet[];
rv = Rasterize/@v;

NearestNeighborGraph[
    rv,
    2,
    VertexLabels->Thread@Rule[rv,v]
]

enter image description here

For another workaround, you can set ImageSize:

NearestNeighborGraph[
 Rasterize[Style[#, 20], ImageSize -> 10] & /@ Alphabet[], 2, 
 VertexLabels -> Automatic]

enter image description here

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