I can't reproduce.  Please do a "set -x" in a terminal, then invoke the "cd /zoo<TAB>", capture the output in a file and attach it here.  Also, is the behavior always the same no matter which dir you're in when invoking the completion?

It seems to work fine if I comment out these definitions in my .bashrc
file:

function _pwd () {
   echo $PWD/
}
function _cd () {
   'cd' "$@" > /dev/null
   _pwd
}
alias cd=_cd

(I like the cd command to tell me the full path name of where I just
wound up, very handy for current directory tracking in an emacs shell).

With those defined, this is what set -x shows:

zooty> set -x
zooty> cd /zoo+ cd cd /zoo cd
bash: cd: cd: Not a directory
+ _pwd
+ echo /home/tom/
/home/tom/

Without those .bashrc defs, a humoungous set of shell gibberish is printed,
but /zoo does, in the end, get expanded to /zooty/

The function bash-completion uses for completing the cd command is also called _cd, and your bashrc definition overrides it with an incompatible one.

There has been some talk upstream to introduce a "namespace" for everything bash-completion uses/defines to make clashes like this less likely to happen (e.g. prefix all of its functions with _bashcomp_), but it is not clear when that might be implemented if at all.

I suggest renaming your _cd function so something else that doesn't clash with bash-completion.  By the way I believe your bashrc setup could be simplified to something like this, no need for aliases or other functions for this purpose:

cd() { builtin cd "$@" > /dev/null ; echo $PWD/ ; }