It looks like that’s not undefined behaviour, but using a non-existent and undefined opcode, which happens to “do something” because the cpu attempts to execute

It looks like that’s not undefined behaviour, but using a non-existent and undefined opcode, which happens to “do something” because the cpu attempts to execute the bits anyway (it doesn’t mask the unused opcodes into no ops or errors). Later variants of the 6502, like the WDC 65C816, did away with this (I believe, either using the opcodes for something defined, or making them no ops, although I’m unsure)