Just to clarify, Philips isn't the only one who says that MOV A, ACC is not a valid 8051 instruction. Intel is another. You can look up the 8051 instruction set references on Philips's or Intel's websites to verify this.

It's possible that something about the operation of the "MOV A, direct" instruction on the original Intel 8051 scrambled the accumulator before writing the memory contents to it. This would lead to MOV A, ACC being an invalid instruction. Anyway, it's not an instruction I plan to use; I was just curious if anyone knew more about it.