console:
[CHAT] §cThe undead horde is approaching...

json:
"wave_message": "§cThe undead horde is approaching...",

A white symbol shows up before invasions, presumably because of the color code symbol. It shows for some players, and some don't get it. It might be something to do with Windows language, but I'm not sure.

I assume it was language because this shows up when Windows is set to Japanese, but not Minecraft.