While I have never seen a throw out bearing that was attached to the clutch disk, I suppose it is possible. If the bearing were supposed to be attached to the clutch, it would mean the bearing had to rotate all the time, and it wouldn’t last long.
If the tractor was stored in a wet location with the clutch pedal pushed in, it might rust together, but that would take extreme circumstances.
More likely is the bearing failed and seized up, then when it was pushed against the clutch, it welded itself to the clutch.
Please post a picture if possible. Hopefully someone with a 354 will add some information about how it is made.