Just put a little grease where the piston end touches the cylinder wall. The binding is a common problem, I posted a video here of somebody that has the trouble. Molly grease is already on the end of the rod from factory.
KHI and the dealers seem to try and avoid any direct answers yet the newer slave cylinders are of a modified type, there should be a recall.
The modified part number is 99999-0610.
I had this very issue last year in march...Kawasaki changed slave cylinder, push rod and input shaft. 3500 miles later in August that year bike lay for a few weeks and the problem happened again. Left it overnight went back out on it issue never came back rest of the season.
Went out today for the first time in 5 months....and 20 minutes on the bike the same thing is happening again. Pissed right off with this bike now.
Glad you managed to sort the issue yourself, I'm looking at doing the same this time as Kawasaki UK are useless. Can you explain in more detail where you put the grease and let me know if the issue has came back since you applied it ?