Unexpected reduction result for λab.a (SUCC b) with Church Numerals

[Xingye-Dujing]

Hello,
I am encountering an unexpected behavior while evaluating the following lambda expression involving Church numerals:

λab.a (SUCC b)

I think the final result should be church numeral 16 and church numeral 64.
Generally, a (SUCC b) should be church numeral (b+1)^a. (The a and b on the right side refer to the corresponding mathematical values of the Church numerals.)

[Xingye-Dujing]

The free variables mentioned below all represent Church numerals, such as a and b;
Furthermore, we stipulate that a prime symbol (') is added to the upper right corner of the free variable to indicate the corresponding mathematical value of the Church numeral:
We require (a (SUCC b))':

1. If there are parentheses, we look at them first. The result of SUCC b is a Church numeral.
2. We know that (a b)' = b'^a'. Therefore, (a (SUCC b))' = (SUCC b)'^a'
3. We know that (SUCC b)' = b' + 1
4. From the above, (b' + 1)^a'