I could not find any operating instructions for that incubator. Mine doesn't work that way. I think you are supposed to set the humidity you want and it automatically mists water in there to reach that level. Since it is a forced air humidity should be the same throughout.
You can try adjusting the setting up and see what happens. It's possible the sensor or something else isn't working right.
That incubator turns the eggs by having rollers on the bottom, many incubators do it that way. You can try setting a reservoir of water in there if the rollers don't cause a problem. I'd want something with a slick bottom to set the reservoir on. That might stabilize it.
I'd probably be getting frustrated with it since it appears to not be working correctly. Good luck!