i++ and (i)++ behave identically. C 2018 6.5.1 5 says:
A parenthesized expression is a primary expression. Its type and value are identical to those of the unparenthesized expression. It is an lvalue, a function designator, or a void expression if the unparenthesized expression is, respectively, an lvalue, a function designator, or a void expression.
The wording is the same in C 1999.