That probably explains it. I rebuild quite a few scooter engines, bike hubs as well as 4x4 gear/transfer g/boxes, only ever use European or Japanese ones from known manufacturers. Next time either measure the bearing, look for the bearing number or take it along with you and buy your bearing from a proper bearing stockists. When I rebuild anything, any dubious bearings are thrown away regardless of condition. One of my own engines had previously been rebuilt with Russian bearings and had done virtually no miles since its rebuild, hadn't even been run in, they still went straight in the scrap pile.
We mainly use SKF or Koyo bearings, a bearing that size for £1.70 won't be the best.
We refuse to fit customer supplied if they've brought in cheap sh1t.