Yes. This is dangerous.
I would suggest a couple of issuess:
1. A LOT of air in the lift cylinder if it falls as soon as you stop lifting. Bleed the lift cylinder
2. If I remember correctly lift is controlled by a proportional lift valve. The valve may be malfunctioning or there could be a problem with the hydraulic manifold - easy check is to swap with a bank from another lift if you have a fleet, the manifolds are expensive, don't fail too often, but it can happen.
3. Have your tech check the hydraulic schematic. I think that there shold be a check valve to stop fluid bleeding through. If so, check it out
4. The main lift cylinder may be bleeding through. raise the mast all the way up with a weight and check for drift down..
5. Call the dealer

Hope that helps