Sorry to say, this is a known bug in Kontakt 5, but it is still not fixed in the newest version (Kontakt 6)
It is a bug in convolution effect.
I am using a workaround in scripting.
In the on Activate event a very short note with a very low velocity is sent to Kontakt 
And here is the script:
var
RHODES : MidiInBlock
WHURLI : MidiInBlock
On Activate
PlayNote(RHODES, C3, 1, 1, 200, 100)
PlayNote(WHURLI, C3, 1, 2, 200, 100)
end