It would be of great help to know which spindle on what deck under which Kubota
Some of the Kubota spindles use tapered rollers & others use deep groove balls
Some have shoulders on the spindle & some have a bearing spacer
Some are retained on the outside by shoulders machined into he housings while others have retaining ring.
Did you use genuine Kubots bearings, OEM bearings from a mower parts retailer or something of unknown quality of evil pay or ammozone
If so vendors and bearing numbers please
The long & short, if the spindle jambs then either the bearings are out of alignment or you are crushing the inner races agains the rolling members.