Item functions %character%.Worn.Size. and %character%.Worn.Weight. will display a popup list of functions including "Sum".
However, if we use any other integer property such as %character%.Worn.ArmorClass. then no popup list appears at all, even though the Sum function is valid here.
It should give the same popup list of functions as Size and Weight.
The integer properties are displayed with a filter icon so I assume that means that %character%.Worn.ArmorClass is filtering the group to only include those that have this property selected, but appending the Sum function changes the meaning of the previous function so that it returns a list of values instead of acting as a filter.