garbage collector doesn't track natively allocated as gray node

[ffwff]

function main() begin
    args = Sys::args()
    inf = ""
    for i=0 to 2 then inf += 'a'
    chars = inf.chars()
    s = 'a' + 'b'
end

main()

The garbage collector doesn't track the array allocated by String::chars as a gray node. valgrind shows 3 errors from 3 contexts.