If it shows up there, but queries to http://188.74.121.175:4153/ (for MR) are unsuccessful, it indicates that the built-in webserver is apparently broken (no longer responds etc).
I think this may be the case.
Ozorvals tested this. The server did not show up on his client when he logged in, he checked the api server list and the server was showing up in there.
Connecting via the IP and port worked fine.
I have noticed too, that if you refresh the list, the online server count often changes. Too frequently I would say in order to be attributed to servers going online/offline.
I also did another test, I used a well known VPN service and logged into Steam (I know Steam does not ike you doing this
but I'm sure that's more for people trying to get discounts on apps that are country specific - don't do this folks you can get your account banned!).
Anyway, whilst using the VPN to test I did notice a lot of servers were missing, pressing refresh changed this list each time, and rather inconsistently. I'm not sure why using a VPN should have this affect. Again, in this situation when connecting via IP worked fine without the server showing in the list.
I hope this info helps.