It really doesn't matter what it is, the trans needs to come out and apart. Then you'll know.
The shaft bearing and the needle thrust bearing were both in pieces in an Ambo tranny that I rebuilt. I couldn't determine which one failed first.
IMO, the main reason for release bearing is, improper free play adjustment. So many times I get bikes in with the cable so tight that the release bearing is always under load. It won't last long like that. Also, the clutch lever spring on the rear of the trans is often missing.