I say possible because maybe this is working as intended. I was doing some experimenting with updating a custom field via the API with the /assets/{AssetId} endpoint. Initially when running it I got this message back:

 "Errors":  
    {
      "Message": "Asset tag is required"
    },
    {
      "Message": "Model is required"
    }
  ]

Adding the Asset Tag and Model ID seemed to resolve both issues. I was also able to update both Notes and a Custom Field (experimenting with how I want to store a bit of info). However, I did NOT expect it to remove the Owner, location, or serial number from the asset. In the grand scheme of things I'll already have that information from earlier in my script I'm working on, so not a huge deal. But it's certainly not what I thought would happen. I would have anticipated only the fields I was referencing to be updated.