I have a 2018 Z Force EX 800. Has anybody had any problems with the steering wheel not staying in place after it has been lowered?

Yes.

In my case, the top pin plunger suffered some corrosion and would not return after being depressed, and so would not lock the steering in place. 

I fixed this by removing the piston, soaking the pin with penetrant, using a vice grip to grab the top of the pin and pulled until the pin worked its way out. Then, using silicon grease on the pin, I forced it in and out until all the corroded bits came out with the grease. I then repeated with 3-in-one oil to clean out the grease until the pin moved freely again. I also wire brushed the pin to remove some roughness and scaling.

It's working fine again.

Thank You! I will do that right now. Appreciate your quick response!

[2015 Z Force 800 53/Trail EPS]

Mine had a different issue and during the fix-process...another that sounds like what you are going through. First, my lever-to-valve release (that's the little button in the center of the shaft) was loose enough that the lever would come out. Adjusting it, which is just loosen the lock nut and screwing-in the shaft a little is the fix...but on my first attempt I didn't leave any play in it so the button was poised to be pushed slightly...and with the wheel set low, it did just that and would move back up to a point where the button didn't have pressure on it. It was just a matter of backing the shaft back out a hair that fixed that. So if your lever doesn't have any free play in it...this is probably what it's doing. An easy fix.

Also, I have a question for the other Z owners out there. Mine when you move the wheel all the way down the u-joint's yokes hit each other because the angle is too much and will actually prevent the steering shaft from turning. True it's so low the wheel is in your lap and probably no one would drive it that way.. but...I was amazed they would allow that to happen for safety reasons. Does anyone else's do this? I have taken it out and ground down where it hits some to reduce this but...it can still happen.