Several commands, such as /seen and /whois no longer correctly tab complete in the most recent versions of EssentialsX. For these particular commands, they would tab complete for a player currently online in prior versions.

List of broken commands (missing tab completion):
/exp give/set
/speed
/unlimited
/setworth
/realname
/seen
/spawner
/whois
/spawnmob
/sudo
/unban
/unbanip

List of broken commands (incorrect tab completion):
/tree
/remove
/setspawn

Can confirm tab complete isn't working for some commands including /seen and /whois. Tab complete does work for other commands such as /tpa, /tp, and /mail.

Can also confirm, not working.

I also have this problem.

Confirmed

Found more commands that do not adhere to the syntax description found at http://wiki.ess3.net/wiki/Command_Reference. I've put these in the OP, but keep in mind that these are only commands that I tested, and therefore by no means an exhaustive list. I'll update the list if you guys find anything else broken.

This has been fixed in 12930cf. You can grab the fix from build 493 on the CI server.