I think you have it diagnosed correctly. It is probably the cylinder before the wing cylinder that is leaking oil past the seal and forcing more oil to the wing cylinder thereby raising it. If the wing cylinder was leaking down it would be the wing cylinder. Since it is raising it is the cylinder before it. If you measure the exposed rod length while it is level and again when the wing raises I bet you will find the inner part of the frame has went slightly deeper.

If it is an old machine......it happens. If it is a new one may have a defective seal to begin with. Seems like when our stuff starts getting 7-10 years old we start replacing either seal kits or just put a new cylinder on, depending on what type cylinder and what the replacement cost is. We often keep a couple of the popular sizes on hand as we have a number of machines that use rephasing cylinders. You can find the 8" stroke ones fairly easily after market but Sunflower is bad about wanting to use 10" stroke and I have not found a source for them except through Agco.

John