EssentialsX version (/essentials):
b530
Server software (/version):
CraftBukkit version git-Spigot-7754231-362ed32 (MC: 1.12.2) (Implementing API version 1.12.2-R0.1-SNAPSHOT)

Jailed player issues non-essentials command, jailMessage of the locale is triggered twice;
Jailed player issues essentials command like /spawn --> results in player X jailed for X seconds
Jailed player doesn't get teleported back to his previous location when he has been released; only the message jailReleasedPlayerNotify pops up.

git-Spigot-f982156-22564a9 (MC: 1.12.2), EssentialsX 2.0.1-b547 - Was unable to replicate issues, they must be resolved. What about you now @FrankHeijden?

@FrankHeijden Could you post your config?

If EssentialsX is configured to listen to the teleport listener and change the /back last location, then this could mean that their last location is being reset when they try to teleport while in jail.

Jailed player issues non-essentials command, jailMessage of the locale is triggered twice; - this issue still persists for both permanent jails and temp jails

Jailed player issues essentials command like /spawn --> results in player X jailed for X seconds - this issue is only with temp jails

Jailed player doesn't get teleported back to his previous location when he has been released; only the message jailReleasedPlayerNotify pops up. - this issue is only with permanent jails.

@FrankHeijden Just to make sure, update to the latest build?
Also, please post the config.yml like was asked, if you would.

Closing due to lack of response; if this is still a problem, please reopen the issue, including your config.yml otherwise we can't help.