This was an issue in Core.

Changes have been made in Git (master):

commit 8498de796fec46cc1ee35f1c8bee6454b133ef30
Author: Michael J Rubinsky <mrubinsk@horde.org>
Date:   Wed Jun 11 18:27:41 2014 -0400

     Bug: 13256  If knl is open, only process the selected node.

  framework/Core/js/prettyautocomplete.js |   12 ++++++++++--
  1 files changed, 10 insertions(+), 2 deletions(-)

http://github.com/horde/horde/commit/8498de796fec46cc1ee35f1c8bee6454b133ef30

Changes have been made in Git (FRAMEWORK_5_2):

commit c30ef30c407b11f8753da413f0df57ce5bcfce17
Author: Michael J Rubinsky <mrubinsk@horde.org>
Date:   Wed Jun 11 18:27:41 2014 -0400

     Bug: 13256  If knl is open, only process the selected node.

  framework/Core/js/prettyautocomplete.js |   12 ++++++++++--
  1 files changed, 10 insertions(+), 2 deletions(-)

http://github.com/horde/horde/commit/c30ef30c407b11f8753da413f0df57ce5bcfce17

Some history:

We needed to make the enter key trigger the currently typed text to be 
processed because often times the enter key triggers submission of a 
form. Otherwise, the currently typed text would be silently discarded.

That being said, it looks like we will have to track the current state 
of the keynavlist (the select list) and disable the enter key when it 
is shown and nothing is selected and cause a selection when it is 
shown and an item is selected.

Search a person under attendees and select it by keyboard with arrow 
down + enter. Now I have two attendees, one with my search word and 
the selected one.

Select it with a mouse click works.