@Jih-Kai_Liang thanks for reporting this. 
I am seeing the same thing on Debian bookworm (i.e. Debian 12, not yet released).
lxd 5.5-37534be running as a snap (regretfully, can’t wait until Debian bug #768073 is resolved and we a have a native non-snap package in the distro)
Creating the symlink like this worked for now:
$ cd /etc/bash_completion.d/
$ sudo ln -sf /snap/lxd/current/etc/bash_completion.d/snap.lxd.lxc .
The lxc con+ command you pasted doesn’t work at all for me though:
$ lxc con+ local etel=snap/core/current/usr/lib/snapd/etelpmoc.sh
Error: unknown command "con+" for "lxc"
Did you mean this?
copy