I believe if affects everyone when this happens.
If it really affects everyone, it indicates that either the server isn't added to the Steam masterlist (that would be a Steam related issue, or more precisely, it would indicate that the Steam masterserver is unreliable), or there might be a problem with the built-in webserver (so the server no longer responds to queries from the clients) 
If it doesn't show up in the API response https://api.rising-world.net/v5/serverlist , it's most likely related to the Steam masterserver (that would tell me that we have to get rid of it in order to fix this issue). 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).
It has impacted different servers for me, without much rhyme or reason. Other servers show up and work just fine. Connecting via IP works on the missing server. And it doesn't seem like they recover without a restart... ie. any time I've seen a missing server after it comes back I've always heard from other players that there was a restart.
Do you mind sending a report if this happens again? 
I.e. if you can't find the server anymore, but you can still connect to it (just send the report after being able to successfully connect to the server, while the server didn't show up in the list). It would help me to find out if the server is either missing in the master list (i.e. a serverside or Steam-related issue), or if the client cannot query the server data.
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!).
