I tried setting the title of my knight rank as [Sir] by issuing the command. /lp group ritter meta addprefix 1 "&c[Sir]". Should work correctly; however, it displays like in the attached image. How do I fix this and make it display normally?

Looks more like a problem with the format of your chat plugin

I'm also having this issue. I know it's not a luckperms issue though, hoping someone could help out with this anyway though!
I'm using essentialsx, and the chat part of the config looks like this.

Sorry by the way, bit of a new comer at this and I'm struggling to find an answer anywhere as I guess it's a simple answer.

You should ask these kinds of questions somewhere else (for example on Spigot forums).
For LuckPerms prefix is simply a permission node and LP has nothing to do with prefixes being displayed.

Uncomment (remove #) the format line in config.yml of EssX and change the value. You should use predefined variables like {PREFIX} or {DISPLAYNAME} as well as formatting codes and other characters to make the chat look exactly the way you want it.