So as far as I can see in the description, the train on a smart station can decide where to go next ("1st, if Signal # is checked and the signal lamp has a valid station # input to it, the train will choose this station.").

So I created 3 stations, A, B, C, set mapping for B and C to 100 and 101. Created a line with A and B (both with timeout of 5s), in the Rules UI enabled "use station mapping" and "Signal #" for A.
For the station A I have a constant combinator sending 101 as "Station #" and "Destination #", the combinator is connected to station's signal lamp.
I would expect the train to go to C, but in the reality it just goes A-B-A-... as if all this smart trickery didn't exist.

What's wrong? How exactly should this feature be used?