async callbacks are not awaited

[TSoli]

So I have a callback that I need to await a transform from tf2 in. Here is a MWE using a regular ros2 subscription that works

import rclpy
from rclpy.node import Node
from sensor_msgs.msg import Image as ImgMsg
from tf2_ros import TransformException
from tf2_ros.buffer import Buffer
from tf2_ros.transform_listener import TransformListener


class TestAsync(Node):
    def __init__(self) -> None:
        super().__init__("test_async")
        self.tf_buffer = Buffer()
        self.tf_listener = TransformListener(self.tf_buffer, self)
        self.create_subscription(
            ImgMsg, "zed2/zed_node/rgb/image_rect_color", self.async_callback, 10
        )

    async def async_callback(self, msg: ImgMsg) -> None:
        self.get_logger().info("Entered callback")
        now = rclpy.time.Time.from_msg(msg.header.stamp)
        try:
            tf = await self.tf_buffer.lookup_transform_async(
                msg.header.frame_id, "odom", now
            )
            self.get_logger().info(f"Got {repr(tf)}")
        except TransformException as ex:
            self.get_logger().info(f"Exception: {ex}")


def main():
    rclpy.init()
    node = TestAsync()
    rclpy.spin(node)
    rclpy.shutdown()

When I switch this MWE to using message_filters like this

        img_sub = message_filters.Subscriber(
            self, ImgMsg, "zed2/zed_node/rgb/image_rect_color"
        )
        img_sync = message_filters.TimeSynchronizer([img_sub], 1)
        img_sync.registerCallback(self.async_callback)

instead of

        self.create_subscription(
            ImgMsg, "zed2/zed_node/rgb/image_rect_color", self.async_callback, 10
        )

I get the error

timeWarning: coroutine 'TestAsync.async_callback' was never awaited 
  cb(*(msg + args))  
RuntimeWarning: Enable tracemalloc to get the object allocation traceback

and the callback does not seem to be called correctly (I see no logs).

This is a toy example for demonstration but in my use case I am trying to use message_filters.TimeSynchronizer to synchronize messages from callbacks for multiple topics. Is there something I am doing wrong or is this a bug?