A completions are not complemented by TAB key, When 'clcompleteopt' option include "menu" and "noinsert", and there is only one match.

[h-east]

How to reproduce:

$ vim -Nu NONE -c 'set clpum clcot=menu,noinsert'

:set clcot=<TAB>

Expected behavior:
menu,noinsert is complemented.

Actual behavior:
Nothing complemented.

Remarks:
It is complemented with the use of the <C-D> instead of <TAB>.
I checked by CUI Vim 7.4.1290 on fedora Linux.

[h-east]

This behavior is expected. See document :h 'clcot'

           menu     Use a popup menu to show the possible completions.  The
                    menu is only shown when there is more than one match and
                    sufficient colors are available.

Insert mode completion with set cot=menu,noinsert is also a similar behavior.