Given that we can't look into your instance, you have to be more specific on your setup. I just created a Linux server on my OOB system and nothing was set on the OS field. So OOB there isn't anything setting OS on Linux servers.
Do you have an integration? Check the transform maps.
Is it done manually and auto filled? Check your client scripts/business rules/flows.
Do you mean something else? Please elaborate.
Please mark any helpful or correct solutions as such. That helps others find their solutions.
Mark
