Usually, the historical reason is that adding the logic to do something well-defined when unexpected prefixes are used is going to cost ten more transistors per

Usually, the historical reason is that adding the logic to do something well-defined when unexpected prefixes are used is going to cost ten more transistors per chip, which is going to add to cost to handle a corner case that almost nobody will try to be in anyway. Far better to let whatever the implementation does happen as long as what happens doesn't break the system. The issue here is their verification of possible internal CPU states didn't account for this one. (There is, perhaps, an argument to be made that the x86 architecture has become so complex that the emulator between its embarrassingly stupid PDP-11-style single-thread codeflow and the embarrassingly parallel computation it does under the hood to give the user more performance than a really fast PDP-11 cannot be reliably tested to exhaustion, so perhaps something needs to give on the design or the cost of the chips).