That is a pretty normal depth reading for being in air, which is good and means that your sensor is working normally if you are getting temperature and pressure readings from an Arduino or Pi.
It looks like you have what is called a “Pixhawk 2.4.8”, which is not what we have tested our products with. That 2.4.8 is a Chinese knockoff and from searching through the ArduPilot forums, one with a lot of issues: 2.4.8 Search Listing.
We use a Pixhawk that is the closest we could find to the original 3DR Pixhawks (they had an unofficial version of 2.4.6).
I think you may have a faulty I2C bus on your Pixhawk. Do you have any other I2C accessories you can try with it?