No tableau guru here, but we fell in to a similar situation.
We made the 'Same Month Previous Year' calculation = 13 rather than having the year datediff in there.
if DATEDIFF('month',[Date],today())=13 then [SALES] end
Still not sure why the Datediff('year' ...) wont work, so might have to go back and investigate.
Hope it helps
Thanks so much! I can't believe I didn't think of that!