Do you have Vault installed?

From the reports I've seen, it looks like LuckPerms and some other perms plugins have issues with getting perms for offline players (not offline mode, but disconnected players), based on how EssX responds in debug mode (permission is false when they try to connect).

Working fine for me. (latest Paper, Essentials and LP, no other plugins.)
Wouldn't be a Vault issue, as (at least with the latest Essentials versions) it doesn't effect permission checks.