`waitForTransform()` throws an exception even though the future should be ready

Bug report

Required Info:

• Operating System: Ubuntu 20.04
• Installation type: Binaries
• Version or commit hash: Foxy
• DDS implementation: rmw_cyclonedds_cpp
• Client library (if applicable): rclcpp

Steps to reproduce issue

I made a simple test package with one node that sends two transforms and one node that requests a transform that can be interpolated from those two.

git clone git@github.com:nnmm/tf2_example.git
cd tf2_example
git checkout 5b837b82b15555f1cd5f35df3987b8e95c172e19 # for reproducibility in case the main branch changes
colcon build
source install/setup.bash
ros2 launch wait_for_transform wait_for_transform.launch.xml \
        executor:=single \
        method:=wft_wait_for \
        request_time:=newest \
        send_first_tf:=during \
        send_second_tf:=after

It doesn't matter whether the second transform is sent during or after the waiting period.
The code that makes the request request looks essentially like this (wft_wait_for method in receiver.cpp):

auto future = tf_buffer_.waitForTransform("a", "b", tf2::TimePointZero, timeout_, callback_);
auto status = future.wait_for(timeout_);
if (status == std::future_status::deferred) {
  RCLCPP_INFO(get_logger(), "Status is deferred");
} else if (status == std::future_status::timeout) {
  RCLCPP_INFO(get_logger(), "Status is timeout");
} else {
  RCLCPP_INFO(get_logger(), "Status is ready, calling future.get()");
  const auto lu_time = tf2_ros::fromMsg(future.get().header.stamp);
}

Expected behavior

The most recent transform is requested, so as soon as the first transform arrives (which happens during the waiting period), I should be able to obtain the transform.

Actual behavior

The future has status ready as you'd hope, but an exception is thrown by get():

[receiver-2] [INFO] [1605268518.458370419] [receiver.callback]: Calling get() ...
[receiver-2] [INFO] [1605268518.458434859] [receiver]: Status is ready, calling future.get()
[receiver-2] terminate called after throwing an instance of 'tf2::LookupException'

Additional information

I haven't done much with futures before, but this seems like the way it could plausibly work, given https://github.com/ros2/geometry2/issues/347 and https://github.com/ros2/geometry2/issues/346 (only guessing since there is no documentation).

The timeout given to waitForTransform doesn't seem to matter anymore, only the timeout given to wait_for().