x64 will break at 00007FF6F0E78FB1, one instruction after the byte was written. So the byte was written at 00007FF6F0E78FAB, with the instruction mov byte ptr ds:[7FF6F0F148E4],al. We trace back again where al is modified. al is the the least significant byte of the rax register, which is used to hold return values of routines (functions). The instruction before this one at 00007FF6F0E78FA6 is a routine call. After exiting a routine call the return value is stored in rax. It is highly likely that the rax register is modified in there.