duplicates a term that lasts after failure

?- T = nice(N),
        (   N = world,
            nb_linkval(myvar, T),
            fail
        ;   nb_getval(myvar, V),
            writeln(V)
        ).
nice(_5314)
T = V, V = nice(N).

?- T = nice(N),
        (   N = world,
            nb_setval(myvar, T),
            fail
        ;   nb_getval(myvar, V),
            writeln(V)
        ).
nice(world)
T = nice(N),
V = nice(world).