Having attempted to farm some kings over the past few days, I have noticed that some kings timers are not working consistently, some are 30 minutes and some are 1 hour.
On day one, the dreadnaught spawned correctly, 30 minutes after being killed, the next 3 times I attempted this, he had not respawned after 30, 35 or 40 minutes, after that I gave up, much the same with the sikuku elite, sometimes he will spawn on the hour, other times he doesn't, I know it is not anyone killing them because I arrive a couple of minutes early.
I am sure there are other kings with these issues, but I have not tried farming any others.