Here is an excerpt from PI ACE user manual on natural scheduling stating that you cannot use the same tag for both input and output; it is simply because any output would then trigger a new calculation and therefore, infinite loop:
"Natural scheduling means that an ACE Context is evaluated whenever one of its trigger PI tags has received a new snapshot event. The trigger tags must be a subset of input tags/aliases used in the ACE Module. Tags/aliases used as both input and output cannot be used as trigger tags as this may easily result in an infinite loop if not handled properly. By default, the calculation result has the same timestamp as the trigger tag event. Natural scheduling is highly accurate; it has the same effect as if the calculation were performed continuously."
It may also be useful for you to read this KB article on the PIACEPoint object and how the whole scheduling and input/output tags work in PI ACE.