eBGP routes with different MED values cannot be imported correctly into RIB

[ryu-s-4]

Hi,

I tried to advertise eBGP routes from two routers (R1 and R2 in the following picture) respectively with different MED values (but with the same destination) to DUT, where GoBGP is running as a routing protocol.
The topology is as show in the following.

<<<
|               |
|               |
|               |
| AS      ---------------
|65001    |     DUT     |
|         ---------------
|           |         |
>>>         |MED      |MED
<<<         |50       |100
|           |         |
|         ------   ------
|AS       | R1 |   | R2 |
|65002    ------   ------
|           |         |
|
>>>

Although the routes are advertised to DUT, it is observed that the routes cannot be imported correctly into RIB, specifically, the MED values are not set correctly for the advertised routes.
the appearance of “gobgp global rib” is as follows.

root@iptun-mic:~# gobgp global rib
   Network              Next Hop             AS_PATH              Age        Attrs
*> 20.0.0.0/24          172.16.4.2           65002                00:00:58   [{Origin: i} {Med: 0}]
*  20.0.0.0/24          172.16.5.2           65002                00:00:54   [{Origin: i} {Med: 0}]
root@iptun-mic:~#
root@iptun-mic:~# gobgp global rib
   Network              Next Hop             AS_PATH              Age        Attrs
*> 20.0.0.0/24          172.16.5.2           65002                00:00:10   [{Origin: i} {Med: 0}]
*  20.0.0.0/24          172.16.4.2           65002                00:00:08   [{Origin: i} {Med: 100}]
root@iptun-mic:~#
root@iptun-mic:~# gobgp global rib
   Network              Next Hop             AS_PATH              Age        Attrs
*> 20.0.0.0/24          172.16.4.2           65002                00:00:04   [{Origin: i} {Med: 0}]
*  20.0.0.0/24          172.16.5.2           65002                00:00:03   [{Origin: i} {Med: 50}]
root@iptun-mic:~#

As shown in the logs, MED values of the either (or both) of the routes are set with 0 randomly, and I have no idea why this happens.
The configuration for DUT is as follows.

# -*- coding: utf-8; mode: toml -*-

[global.config]
  as = 65001
  router-id = "10.10.10.10"

  [global.apply-policy.config]
    export-policy-list = ["policy-ipiptun-prefix"]
    default-export-policy = "accept-route"

[[neighbors]]
  [neighbors.config]
    neighbor-address = "172.16.4.2"
    peer-as = 65002

  [[neighbors.afi-safis]]
    [neighbors.afi-safis.config]
      afi-safi-name = "ipv4-unicast"

[[neighbors]]
  [neighbors.config]
    neighbor-address = "172.16.5.2"
    peer-as = 65002

  [[neighbors.afi-safis]]
    [neighbors.afi-safis.config]
      afi-safi-name = "ipv4-unicast"

[zebra]
  [zebra.config]
    enabled = true
    url = "unix:/var/run/frr/zserv.api"
    version=5

I wish I would know whether this is a bug or configuratoin mistake.
Thank you in advance!