It's because (fn args ...) currently compiles to (lambda args ...), so args becomes a Scheme list instead of an Arc list.
It's hard to see the difference because the Arc runtime mostly treats '() as a synonym for 'nil. But keys to tables aren't converted, so it is possible to get an observable result out of the bug this way:
arc> (let a (table)
(= (a ((fn args args) 1 2)) 'hi)
(a '(1 2)))
nil
