My own thoughts which are just that, is that the unique fit, coolant plumbing and components of same on BMW's well proven B48 variant collectively cause the issue. Maybe under sudden load, steam is allowed to form probably in the integral to head exhaust manifold. When the thermostat in the radiator return fully opens, such steam pockets enter top of rad and cause mechanical shock from the thermal shock due to sudden cooling.
Cars driven on the track where stat is open most of the time or at the other extreme when gently driven will likely never experience this issue, explaining why not all rads fail and wasn't noticed in original testing.
Certainly mechanical shock in plumbing is well demonstrated at home when a tap is turned off suddenly and a poorly supported copper pipe rattles or bangs, which can turn to leaks at any adjacent joint over time.
