From your description, this seems to be the expected behavior. Objects welded or connected to the user’s character will become owned by that player, bringing the sensor with it. As a general rule, properties of instances do not automatically replicate back to the server (there are some exceptions but this is true for most properties). To get the values on the server, you will need to manually synchronize the client and server states via a RemoteEvent.
Eventually, the Server Authority project ([Early Access] Server Authority) will make these issues much easier to deal with. Let me know if you have any questions. Thanks.