API update method on object replace the lists inside it

Hello, 

I'm using version v17.5 and I'm trying to update the SourceZones and DestinationZones by adding another zone to each one, but when I try to do that, the old data disappears. 

I'm using the regular request and it's working great if I want to replace it, so I know its not the request itself.

Can I update and not replace the data? Is it even possible with the API?

Please help me