Just installed it on my Nexbox and KI pro, and every thing works fine. Think you must have done something wrong.
There can not really do anything wrong.
Clean Install: AV (Composite) output only works up to LE Logo tested several times
CoreELEC-S905.arm-9.0-devel-1523108117, AV out does not work after LE logo, the picture remains black.
Under LE 18.104.22.168 no Problem.