Check to make sure the config.wtf file, located withing your wow\retail\wtf\ folder is not write protected by right clicking on it=>properties=>uncheck read only=>okay
Also, verify that you aren’t using any external programs like Overwolf or Onedrive that might be automatically overwriting the config with some kind of backup version of it.
If it’s not either of those, it could be the WoW CVAR that handles replicating variables between the client and the server, but I can’t think of it off the top of my head other than to do a full CVAR reset by running the command /console cvar_default but that seems a bit heavy-handed for this case. If I remember the variable name, I’ll edit my post with it. Like I want to say it’s something like cvarsyncronize or something like that.
EDIT: Found it, it’s /console synchronizeSettings 1 try running that and then follow it up with a /reload to make sure the setting is saved all the way.