If you have a grid where 2 celestials (let’s call them gates) meet, if you place an Upwell on this spot where 2 grids meet then tethering is only possible when on the grid where the Upwell is more placed on. On the other grid and still at 0KM to the Upwell, you cannot tether.
Can this be fixed? Probably not, I doubt it’s a database change 
I think this potentially means with grid control, you can possibly break Upwell tether.