I would install a manual shut off valve in the feed line. Make sure the valve is open & raise the forks, then close the valve & check for creep.

If creep disappears, you have a control valve issue. If it's still present, then a cyl issue is your cause.

At that point, if a cyl issue is present, then install shut off valves to the secondary cyls (if present), raise the forks again (no need to raise high enough to engage the secondary channel) & then shut off one cyl at time to check leakage of that cyl. If creep is still present after testing both secondary cyl's, the primary cyl needs service.

Even if it's just the "hydraulic fuse" as Ed stated, I'd remove the cyl & have it serviced by a hyd shop for safety's sake.