[erlang-questions] state_timeout reset in gen_statem

[Nato]

I may be muddled, but as far I can observe, my 
code seems correct, but am getting unexpected
behavior from OTP 20's gen_statem behavior.

I have a state that times out in, say, 3 seconds.
I have a `bump' routine that casts to that state, 
and subsequently set it again to 3 seconds. In my
tests, the initial timer holds fast, and despite 
my debugging efforts, the original timer isn't
reset as the documentation says is ought to in
the gen_statem manual (state_timeout() type 
section) :

    `...

    Setting this timer while it is running will
    restart  it  with the  new time-out  value.
    Therefore it is possible to cancel this
    time-out by setting it to infinity.

    ...'

Some relevant code as follows:

    ...
    waiting({call, From}, {entry, Key}, #session{}=Ld) ->
        {ok, Ld1} = handle(start_session, {Key, Ld}),
        Reply     = {reply, From, ok},
        Timeout   = session_duration(alive),
        {next_state, alive, Ld1, [
          Reply, {state_timeout, Timeout, hard_stop}]};
    ...

    alive(cast, bump, #session{}=Ld) ->
        ok = handle(bump_session, Ld),
        Timeout = session_duration(alive),
        {keep_state, Ld, [Timeout]};
    ...

    session_duration(alive) ->
    timer:hms(0, 0, 3);
    ...

Indeed, the new `bump' cast, never resets the
original, extending the timeout to a fresh 3
second timeout.

On a separate note, any best practices for putting
together test code that juggles timeouts to
validate such things, would be appreciate.