Take them back, I did and got a refund and now buy from anyone else

I had a similar problem so purchased some silicon carbide ones from Amazon a few months ago, they were excellent with barely detectable joints.

EDIT

Link not working so search on the following

Sruhrak Sanding Belt 100 X 915mm- 12 Pcs Silicon Carbide Sanding Belts

I am in the process of upgrading my old Clarke CS4, the motor let the nasty smoke out (I’ll put a couple of pics up when I’ve finished it).

I ordered a pack of 10 Zirconium belts from Abtec Abrasives a long time ago, about 10 years (I think it was their own brand or unbranded). I don’t know what their prices are like now but it was a really good deal then. They not only did over 2 1/2 thousand Rhodesian teak 9″x3″ hardwood parquet flooring blocks on all 4 edges and the face I had for my house build but I am still using them, I can not fault them.

Be careful using it as a belt grinder, it will do it but go easy on it. Maybe your motor is more powerful than the 1/3hp that was on the old one I acquired but  if you slow it down too much when grinding it will switch over to the start windings and will soon burn out .. which is I think what happened to mine unless it was just that it was probably as old as me!

Another plus 1 for Abtec I have used them for years and very satisfied with the products.