I've seen this happen before in a zapi controlled system.
There is a setting in the hydraulic setup that is like a ramp speed or a delay for the lowering control. We had to go into several trucks when they first came out and adjust this setting so it would stop quicker when the control lever was released. For some strange reason the factory had them setup with a delay and it was causing alot of complaints with the customers because the forks would not stop immediatly when the lever was released.

It could also as suggested be a cylinder problem, air in the line or even a slow or sticky solenoid valve. But if it is consistent it may just be a control setting.

just some things to consider while troubleshooting