In a LAN world, all OP players can use commands, but the UI and autocompletion are only accessible by the host player

Probably due to tab-completion not being implemented for multiplayer yet.

It doesn't work in multiplayer yet. You can type out commands as usual, you just won't get tabcompletions or any UI for it yet.

Source

Is this still an issue in 18w01a?

Seems to be fixed.