Bug Report

Type /ver NametagEdit and post the output.
4.4.0

What version of Bukkit/Spigot/PaperSpigot are you using? Type /ver
CraftBukkit version git-Spigot-6a14ca4-1215188 (MC: 1.14.4) implementing API version 1.14.4-R0.1-SNAPSHOT

What plugins are you using? Type /plugins
CoreProtect, dynmap, WorldEdit, ViaVersion, WorldBorder, NametagEdit, Essentials, ViaBackwards, VexCore, VexKingdoms

Please explain your issue. How do you trigger it?
It is triggered in a PlayerJoinEvent which is triggered correctly.

Are there any errors in the console? Please use: https://pastebin.com
Nope.

Using commands to change my prefix works fine after enabling LongTags.
EDIT: For anyone else with this issue, you can just use Bukkit.dispatchCommand(Bukkit.getConsoleSender(), "nte player playername prefix yourprefix") for now.