The parentheses always start a subshell. What's happening is that bash detects that sleep 5 is the last command executed by that subshell, so it calls exec instead of fork+exec. The sleep command replaces the subshell in the same process.
When you add something else after the call the sleep, the subshell needs to be kept around, so this optimization can't happen.
When you add something else before the call to sleep, the optimization could be made (and ksh does it), but bash doesn't do it (it's very conservative with this optimization).