I had the same problem with my 2017 X584---3 times!! Each time the problem occurred after the mower had been run for 2 hrs or so. The first time the JD service man stopped a couple of hours later and could not find a problem. The 2nd time I removed the battery to get better access to the linkage and found nothing to explain what was causing the problem. After an hour or so of not being able to find a problem, I resigned myself to another $200 or so for JD to fix it I put the battery back in and suddenly everything again worked properly. After 2 or 3 additional 2 hr. mowings the lift again failed to work. Removed the battery again and again checked the linkage for binding and found everything operated freely all the way down to the hydraulic valve. It is difficult to reach but I could just barely reach the "plunger" at the top of the valve. I managed to move the "plunger" around a little then cleaned everything up as best I could and sprayed every moving contact point in the linkage with WD-40. So far everything seems to work fine and the linkage operates more freely. Don't know if that's a fix that will last but initial results are encouraging.
I noticed that the area around the top of the valve tends to gather dust and when the deck is lowered the valve is extended. Maybe the dust gathers on the extended valve stem and keeps the valve stem from pushing in to raise the deck.
Tug