Clojure vector-dependent behavior

Clojure assoc

applied to vector seems to have inconsistent behavior

When the index is present in the vector, replace the value

(assoc [1 2 3 4 5] 3 42) => [1 2 3 42 5]

      

        
        
        
      

    

When the index is close to the last, the vector grows ( conj

equivalent)

(assoc [1 2 3 4 5] 5 42) => [1 2 3 4 5 42])

      

        
        
        
      

    

Otherwise IndexOutOfBoundsExcpetion

selected

although this is useful in some cases, for example reduce assoc

it can lead to subtle bugs in the program

Is there expected behavior or perhaps a bug in the associated for the vector?