data.table doesn't modify by reference if the object is freshly loaded from file?

data.table doesn't modify by reference if the object is freshly loaded
from file?

It seems that if a data.table is freshly loaded, a function containing :=
wouldn't modify by reference.
Can anyone reproduce it? Is it a bug?
test<-function(data){data[,ppp:=1]}
a<-data.table(x=1:2)
save(a,file="ttt")
load("ttt")
test(a) # show ppp
a # doesn't have ppp
b<-data.table(x=1:2)
test(b) # show ppp
b # has ppp