There is no bug here I think.
If you set
you get then (switchin on InputForm with FI; ) :
In:= FI; SUNDelta[i,i]//SUNSimplify
Out= -1 + SUNN^2
(* as it should be *)
SUNN denotes the number of colors.
Trick[SUNDelta[a, a]] yields (SUNN^2 -1).
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