I had the same problem. It has something to do with the filename extension. Export your new KF8 mobi file to disk, rename the *.mobi file to *.azw3 and copy it to the KK. For me, the new KF8 files then open in KK and look very nice BTW :-)
I haven't found a way in Calibre to do this automatically, but given the recency of the KK update, this is not very surprising.