Essentials completely ignores group formats and uses the standard format even for OPs
Pex + EssentialsX config
https://pastebin.com/HSqsG2u3

Do you have vault installed?

Yup
List of Plugins:
AuthMe-5.2-spigot.jar
AutoMessage.jar
Broadcaster%202.0.0.jar
Clearlag.jar
CustomServerMessages.jar
DynmapPlotMe.jar
EssentialsX-2.0.1.jar
FastLogin.jar
FirstJoinPlus.jar
HolographicDisplays.jar
HomeSpawnPlus.jar
LogBlock.jar
LWC.jar
MakeYourOwnCommands.jar
MaxBans.jar
Modifyworld.jar
Multiverse-Core-2.5-b717.jar
Multiverse-Portals-2.5.jar
PermissionsEx-1.23.4.jar
PlotMe-Core.jar
PlotMe-DefaultGenerator.jar
PowerMotD.jar
ProtocolLib.jar
Reporter.jar
ServerTutorial-2.2.3.jar
SimpleHelpTickets.jar
SpamGuard.jar
Vault.jar
Vote.jar
worldedit-bukkit-6.1.3.jar
worldguard-6.2.jar

You need EssentialsXChat from the CI server.