The issue is that the C memory model allows more behaviours than the memory model of x86-64 processors. You can thus write code which is incorrect according to the C language specification but will happen to work on x86-64 processors. Moving to arm64 (with its weaker memory model than x86-64) will then reveal the latent bug in your program.

And “happen to work on x86-64 processors” also will depend on the compiler. If you write

  *a = 1;
  *b = 'p';

x86-64 will never do such a swap, but x86-64 compilers might.

If you write

  *a = 1;
  *b = 2;