After maintenance today, a number of people in our guild including myself are showing our previous server name in the guild roster and guild chat. Deleting the Cache folder in _retail fixed this for myself but other players in the guild still show their previous names and servers for me. This is definitely an issue caused by whatever maintenance actions were taken today, as we have been on the new server for a number of weeks.
This issue is getting hilariously worse. This specific character I’m posting with now shows in our guild roster as on Turalyon, a full two server transfers ago.
Confirmed with guildmates that others are also seeing the issue. Affected characters seem to get resolved once the user logs on to that character once, presumably because this updates the server-side cache of that character’s current name and realm. Not sure what you did for maintenance today Blizz but it was a doozy. 
This is still occurring and all of my guildmates are seeing it. It makes it difficult to see who is actually online because any affected character will show at the bottom of the roster in an “always online”-like state.
Only thing I can think of is this is somehow related to the fact our guild is a recent transfer (within the last 6 weeks).
Here’s a screenshot of just how silly this is. We are a guild on Dalaran now.
https://imgur.com/a/lUEkMam
I asked a friend on Illidan to check his guild and he doesn’t see an issue, so this seems to be a problem affecting relatively recent transferred guilds.
EDIT 2: My alt guild is also experiencing the issue as a recently-transferred guild.