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@gbj_tester
Because there is just one coarse pitch screw holding the whole thing together, and also trying to bend a pretty thick piece of aluminium too. They always slipped when they were new, they always slip now, and they will always slip in the future. Put some carbon assembly paste in there to give it the best chance, lube the threads and under the nut flange/bolt head, and use a torque wrench to guide you into that sweet spot between tight enough to hold it most of the time and not so tight that you snap the bolt. The fasteners are made of cheese, so I'd be cautious about going past 12Nm*. Or just buy a modern 4-bolt stem - there's a reason why designs change over time.
*If it was supposed to have a bit of rubber in the kerf, it sounds like a XA, in which case you can replace the binder bolt with a high quality (12.9) M8x1.25 screw from a hardware store, which will take more torque to break than the stem body will stand...