sum is in the noun form (see a leading % in the lisp expression)
(%i76) expr: sum(x[i], i, 1, n);
n
====
\
(%o76) > x
/ i
====
i = 1
(%i77) :lisp $expr
((%SUM SIMP) (($X SIMP ARRAY) $I) $I 1 $N)
You need to tell ev to evaluate nouns
(%i77) ev(expr, n = 2, x[1] = 5, x[2] = 3, nouns);
(%o77) x + x
2 1
and request an extra post-evaluation
(%i78) ev(expr, n = 2, x[1] = 5, x[2] = 3, nouns, eval);
(%o78) 8