I suggest that there is damage inside one of the cylinders. The piston has detached from the rod, and all of the oil for the bucket-tilt circuit just flows past through the rod-hole in the piston. It probably did not disassemble itself nicely; piston messed up, seals chewed, etc.
When you rev the engine to wide-open-throttle, you get max oil-flow. Whatever failure is there is slightly constricting that max oil flow, and enough pressure is applied to the other cylinder to roll the bucket back. When you reduce engine rpm - and oil flow - the pressure drops off and gravity rolls the bucket back down.
If you are really horse-shoe-up-the-@#$$ lucky, you could take the cylinder apart, put the nut back on the rod through the piston, and be back in operation. In reality, whatever has failed inside a cylinder may have scored the bore, bent the rod, stripped off the rod threads, etc, etc.
I suggest acquiring two new replacement cylinders for the bucket-tilt cylinders. Theoretically you could replace just one cylinder, but slight difference in retracted length or extended length may put a twist-force on the bucket or damage the cylinders. In Canada, the go-to for such things is Princess Auto. I presume theres a similar supplier in the US.
On my L3130, the "head" of a bucket tilt cylinder had stripped threads and needed replacement. That part alone was like $180 from the dealer. The "kit" of seals was a further $200. For an extra $20 I could have purchased two new cylinders from Princess Auto.
I suspect that parts pricing between repair/replace will be similar in your area.
First step is to take out the bucket-end pins of one cylinder at a time. Then operate the circuit and observe what the cylinder without the pin in it does. It should take you two minutes to figure out which cylinder is pooched. Then remove and disassemble the cylinder that doesn't work. Hopefully your horseshoe is in-place and you just put the rod back into the piston and put the nut on and reassemble.
Which presents a question... if the nut has come off the piston-end of the rod, and you disassemble the cylinder and only the rod comes out... that leaves the piston, with the nut and washer rattling behind it, stuck in the cylinder... how do you get the piston out?
Good luck.
